Ionizing radiation can be divided into photon and corpuscular. Photon radiation includes electromagnetic vibrations, to the corpuscular - particle flow. The concepts of “electromagnetic”, “quantum”, “photon” radiation can be considered equivalent.

The type of interaction of photons with atoms of matter depends on the energy of the photons. To measure the energy and mass of microparticles, an off-system unit of energy is used - electron-volt. 1 eV is the kinetic energy acquired by a particle carrying one elementary charge under the influence of a potential difference of 1 V. 1 eV = 1.6 x 10 19 J. Multiple units: 1 keV = 10 3 eV; 1 MeV = 10 6 eV.

According to modern concepts, charged particles (α-, β-particles, protons, etc.) ionize matter directly, and neutral particles (neutrons) and electromagnetic waves (photons) are indirectly ionizing. The flow of neutral particles and electromagnetic waves, interacting with matter, causes the formation of charged particles, which ionize the medium.

2.1. PHOTON AND CORPUSCULAR RADIATION

Electromagnetic radiation. In radiation therapy, X-ray radiation from x-ray therapy devices, gamma radiation from radionuclides, and high-energy bremsstrahlung (X-ray) radiation are used.

X-ray radiation- photon radiation, consisting of bremsstrahlung and (or) characteristic radiation.

Bremsstrahlung- short-wave electromagnetic radiation that occurs when the speed (braking) of charged particles changes when interacting with atoms of the braking substance (anode). The wavelengths of bremsstrahlung X-ray radiation do not depend on the atomic number of the bremsstrahlung substance, but are determined only by the energy of the accelerated electrons. The spectrum of bremsstrahlung is continuous, with a maximum photon energy equal to the kinetic energy of the braking particles.

Characteristic radiation occurs when the energy state of atoms changes. When an electron is knocked out of the inner shell

of an atom by an electron or photon, the atom goes into an excited state, and the vacated space is occupied by an electron from the outer shell. In this case, the atom returns to its normal state and emits a quantum of characteristic x-ray radiation with an energy equal to the energy difference at the corresponding levels. Characteristic radiation has a linear spectrum with wavelengths specific for a given substance, which, like the intensity of the lines in the characteristic spectrum of X-ray radiation, are determined by the atomic number of the element Z and the electronic structure of the atom.

The intensity of bremsstrahlung is inversely proportional to the square of the mass of the charged particle and directly proportional to the square of the atomic number of the substance in the field of which charged particles are decelerated. Therefore, to increase the photon yield, relatively light charged particles are used - electrons and substances with a high atomic number (molybdenum, tungsten, platinum).

The source of X-ray radiation for the purposes of radiation therapy is the X-ray tube of X-ray therapy devices, which, depending on the level of generated energy, are divided into close-focus and remote. X-ray radiation from close-focus X-ray therapy devices is generated at an anode voltage of less than 100 kV, and from remote ones - up to 250 kV.

High energy bremsstrahlung radiation, like bremsstrahlung x-rays, it is short-wave electromagnetic radiation that occurs when the speed of charged particles changes (braking) when interacting with target atoms. This type of radiation differs from x-rays in its high energy. Sources of high-energy bremsstrahlung radiation are linear electron accelerators - LUEs with bremsstrahlung energy from 6 to 20 MeV, as well as cyclic accelerators - betatrons. To obtain high-energy bremsstrahlung radiation, the deceleration of sharply accelerated electrons in vacuum accelerator systems is used.

Gamma radiation- short-wave electromagnetic radiation emitted by excited atomic nuclei during radioactive transformations or nuclear reactions, as well as during the annihilation of a particle and an antiparticle (for example, an electron and a positron).

Sources of gamma radiation are radionuclides. Each radionuclide emits γ-quanta of its specific energy. Radionuclides are produced in accelerators and nuclear reactors.

The activity of a radionuclide source is understood as the number of atomic decays per unit time. Measurements are made in Becquerels (Bq). 1 Bq is the activity of a source in which 1 decay occurs per second. The non-systemic unit of activity is the Curie (Ci). 1 Ci = 3.7 x 10 10 Bq.

Sources of γ-radiation for external and intracavitary radiation therapy are 60 Co And 137 Cs. The most widely used drugs 60Co with photon energy averaging 1.25 MeV (1.17 and 1.33 MeV).

For intracavitary radiation therapy, 60 Co is used,

137 Cs, 192 Ir.

When photon radiation interacts with matter, the phenomena of the photoelectric effect, the Compton effect, and the process of formation of electron-positron pairs occur.

Photo effect consists in the interaction of a gamma quantum with a bound electron of an atom (Fig. 10). In photoelectric absorption, all the energy of the incident photon is absorbed by the atom from which the electron is knocked out. After the emission of a photoelectron, a vacancy is formed in the atomic shell. The transition of less bound electrons to vacant levels is accompanied by the release of energy, which can be transferred to one of the electrons in the upper shells of the atom, which leads to its emission from the atom (Auger effect), or transformed into the energy of characteristic X-ray radiation. Thus, during the photoelectric effect, part of the energy of the primary gamma quantum is converted into the energy of electrons (photoelectrons and Auger electrons), and part is released in the form of characteristic radiation. An atom that has lost an electron turns into a positive ion, and the knocked-out electron - a photoelectron - at the end of its run loses energy, attaches to a neutral atom and turns it into a negatively charged ion. The photoelectric effect occurs at relatively low energies - from 50 to 300 keV, which are used in x-ray therapy.

Fig. 10. Photo effect

Rice. eleven. Compton effect

Compton effect (incoherent scattering) occurs at photon energies from 120 keV to 20 MeV, that is, with all types of ionizing radiation used in radiation therapy. With the Compton effect, an incident photon, as a result of an elastic collision with electrons, loses part of its energy and changes the direction of the initial movement, and a recoil electron (Compton electron) is knocked out of the atom, which further ionizes the substance (Fig. 11).

The process of converting the energy of a primary photon into the kinetic energy of an electron and positron and into the energy of annihilation radiation. The quantum energy must be greater than 1.02 MeV (twice the rest energy of the electron). This interaction of quanta with matter occurs when patients are irradiated at high-energy linear accelerators with a high-energy beam of bremsstrahlung radiation. The photon disappears in the Coulomb field of the nucleus (or electron).

Rice. 12. Formation of electron-positron pairs

In this case, the entire energy of the incident photon minus the rest energy of the pair is transferred to the resulting pair. Electrons and positrons arising during the absorption of gamma quanta lose their kinetic energy as a result of ionization of the molecules of the medium, and upon meeting they annihilate with the emission of two photons with an energy of 0.511 MeV each (Fig. 12).

As a result of the above processes of interaction of photon radiation with matter, secondary photon and corpuscular radiation (electrons and positrons) arises. The ionization ability of particles is much greater than that of photon radiation. When alternating the processes of formation of electron-positron pairs and bremsstrahlung radiation, a huge number of photons and charged particles are created in the medium, the so-called avalanche of radiation, which decays with a decrease in the energy of each newly formed photons and particles.

The interaction of X-ray radiation with matter is accompanied by its ionization and is determined by two main effects - photoelectric absorption and Compton scattering. When high-energy bremsstrahlung radiation interacts with matter, Compton scattering occurs, as well as the formation of ion pairs, since the photon energy is greater than 1.02 MeV.

The intensity of photon radiation from a point source varies in space in inverse proportion to the square of the distance.

Corpuscular radiation- flows of charged particles: electrons, protons, heavy ions (for example, carbon nuclei) with energies of several hundred MeV, as well as neutral particles - neutrons. Irradiation using a stream of particles has now come to be called hadron therapy. To hadrons (from the Greek word hadros- “heavy”) includes nucleons, their protons and neutrons, as well as π -mesons, etc. Sources of particles are accelerators and nuclear reactors. Depending on the maximum energy of the accelerated protons, accelerators are conventionally divided into 5 levels, with accelerators of the 5th level with Ep > 200 MeV (meson factories)

are used for the production of individual radionuclides. As a rule, the production of these radionuclides in cyclotrons of another level is impossible or ineffective.

High Energy Electron Beam is generated by the same electron accelerators as when receiving bremsstrahlung radiation. Electron beams with energies from 6 to 20 MeV are used. High energy electrons have great penetrating power. The average free path of such electrons can reach 10-20 cm in the tissues of the human body. The electron beam, absorbed in the tissues, creates a dose field in which maximum ionization is formed near the surface of the body. Beyond the ionization maximum, the dose decreases quite rapidly. Modern linear accelerators have the ability to regulate the energy of the electron beam and, accordingly, create the required dose at the required depth.

Neutron - a particle that has no charge. The processes of interaction of neutrons (neutral particles) with matter depend on the energy of neutrons and the atomic composition of the substance. The main effect of thermal (slow) neutrons with an energy of 0.025 eV on biological tissue occurs under the influence of protons formed in the reaction (n, p) and losing all their energy at the place of birth. Most of the energy of slow neutrons is spent on excitation and splitting of tissue molecules. Almost all the energy of fast neutrons with energies from 200 keV to 20 MeV is lost in the tissue during elastic interaction. Further release of energy occurs as a result of ionization of the medium by recoil protons. High linear neutron energy density prevents the repair of irradiated tumor cells.

Another type of neutron exposure is neutron capture therapy, which is a binary radiotherapy method that combines two components. The first component is the stable boron isotope 10 B, which, when administered as part of the drug, can accumulate in the cells of certain types of brain tumors and melanomas. The second component is the flux of low-energy thermal neutrons. Heavy, high-energy charged particles formed as a result of the capture of a thermal neutron by the 10 B nucleus (boron decays into lithium atoms and α-particles) destroy only cells located in close proximity to boron atoms, almost without affecting adjacent normal cells. In addition to boron, the use of drugs containing gadolinium is promising in neutron capture therapy. For deep-lying tumors, it is promising to use epithermal neutrons in the energy range 1 eV - 10 keV, which have a high penetrating ability and, slowing down in the tissue to thermal energies, allow neutron capture therapy of tumors located at a depth of 10 cm. Obtaining high thermal and epithermal fluxes neutrons are produced using a nuclear reactor.

Proton - a positively charged particle. The method of irradiation at the “Bragg peak” is used, when the maximum energy of charged particles is released at the end of the path and is localized in a limited volume of irradiation.

my tumor. As a result, a large dose gradient is formed on the surface of the body and in the depths of the irradiated object, after which a sharp attenuation of energy occurs. By changing the energy of the beam, it is possible to change the location of its complete stop in the tumor with great accuracy. Proton beams with an energy of 70-200 MeV and the technique of multifield irradiation from different directions are used, in which the integral dose is distributed over a large area of ​​surface tissue. When irradiating at the synchrocyclotron at PNPI (St. Petersburg Institute of Nuclear Physics), a fixed energy of the extracted proton beam is used - 1000 MeV and the continuous irradiation technique is used. Protons of such high energy easily pass through the irradiated object, producing uniform ionization along their path. In this case, little scattering of protons occurs in the substance, so the narrow proton beam with sharp boundaries formed at the entrance remains almost as narrow in the irradiation zone inside the object. As a result of the use of continuous irradiation in combination with the rotational irradiation technique, a very high ratio of the dose in the irradiation zone to the dose on the surface of the object is ensured - about 200:1. A narrow proton beam with a half-intensity size of 5-6 mm is used to treat various brain diseases, such as cerebral arteriovenous malformations, pituitary adenomas, etc. Damaging effect carbon ions turns out to be several times higher in the Bragg peak than that of protons. Multiple double breaks of the DNA helix of atoms of the irradiated volume occur, which after this can no longer be restored.

π -Mesons- spinless elementary particles with a mass whose value is intermediate between the masses of an electron and a proton. π-Mesons with energies of 25-100 MeV travel the entire path through tissue practically without nuclear interactions, and at the end of the path they are captured by the nuclei of tissue atoms. The act of absorption of a π-meson is accompanied by the emission of neutrons, protons, α-particles, Li, Be, etc. ions from the destroyed nucleus. The active introduction of hadron therapy into clinical practice is currently hampered by the high cost of technological support for the process.

The advantages of using high-energy radiation for the treatment of malignant tumors located at depth are, with increasing energy, an increase in the deep dose and a decrease in the surface dose, higher penetration with an increase in the relative deep dose, and a smaller difference between the absorbed dose in bones and soft tissues. With a linear accelerator or betatron, there is no need to bury the radioactive source, as when using radionuclides.

When carrying out brachytherapy and systemic radionuclide therapy, α-, β-, γ-emitting radionuclides are used, as well as sources with mixed, for example γ- and neutron (n), radiation.

α -Radiation- corpuscular radiation consisting of 4 He nuclei (two protons and two neutrons) emitted during the radioactive decay of nuclei or during nuclear reactions and transformations. α-Particles are emitted during the radioactive decay of elements heavier than lead or are formed in nuclear

reactions. α-Particles have high ionizing ability and low penetrating ability, and carry two positive charges.

Radionuclide 225 Ac with a half-life of 10.0 days in combination with monoclonal antibodies is used for radioimmunotherapy of tumors. In the future, the use of radionuclide 149 Tb with a half-life of 4.1 hours for these purposes. α-Emitters began to be used to irradiate endothelial cells in the coronary arteries after operations - coronary artery bypass grafting.

β -Radiation- corpuscular radiation with a continuous energy spectrum, consisting of negatively or positively charged electrons or positrons (β - or β + particles) and arising from the radioactive β-decay of nuclei or unstable particles. β-Emitters are used in the treatment of malignant tumors, the localization of which allows direct contact with these drugs.

Sources of β-radiation are 106 Ru, β - emitter with an energy of 39.4 keV and a half-life of 375.59 days, 106 Rh, β - emitter with an energy of 3540.0 keV and a half-life of 29.8 s. Both β-emitters 106 Ru + 106 Rh are included in ophthalmic applicator kits.

The β-32P emitter with an energy of 1.71 MeV and a half-life of 14.2 days is used in skin applicators to treat superficial diseases. The radionuclide 89 Sr is an almost pure β-emitter with a half-life of 50.6 days and an average β-particle energy of 1.46 MeV. A solution of 89 Sr - chloride is used for the palliative treatment of bone metastases.

153 Sm with β-radiation energies of 203.229 and 268 keV and with γ-radiation energies of 69.7 and 103 keV, a half-life of 46.2 hours, is part of the domestic drug samarium-oxabiphora, intended to affect metastases in bones, and also used in patients with severe pain in the joints due to rheumatism.

90 Y, with a half-life of 64.2 hours and a maximum energy of 2.27 MeV, is used for a variety of therapeutic purposes, including radioimmunotherapy with labeled antibodies, treatment of liver tumors and rheumatoid arthritis.

Radionuclide 59 Fe as part of a tableted radiopharmaceutical is used at the Russian Scientific Center of Radiology (Moscow) for the treatment of patients with breast cancer. The principle of action of the drug, according to the authors, is the distribution of iron through the bloodstream, selective accumulation in the cells of tumor tissue and exposure to β-radiation. 67 Cu with a half-life of 2.6 days is combined with monoclonal antibodies for radioimmune therapy of tumors.

186 Re in the composition of the drug (rhenium sulfide) with a half-life of 3.8 days is used to treat joint diseases, and balloon catheters with sodium perrhenate solution are used for endovascular brachytherapy. It is believed that there is a prospect for the use of a 48 V β + -emitter with a half-life of 16.9 days for intracoronary brachytherapy using an arterial stent made of a titanium-nickel alloy.

131 I is used in the form of solutions for the treatment of thyroid diseases. 131 I decays with the emission of a complex spectrum of β- and γ-radiation. Has a half-life of 8.06 days.

X-ray and Auger electron emitters include 103 Pd with a half-life of 16.96 days and 111 In with a half-life of 2.8 days. 103 Pd in ​​the form of a closed source in a titanium capsule is used in tumor brachytherapy. 111 In is used in radioimmunotherapy using monoclonal antibodies.

125 I, which is a γ-emitter (a type of nuclear transformation - electron capture with the conversion of iodine into tellurium and the release of a γ-quantum), is used as a closed microsource for brachytherapy. Half-life - 60.1 days.

Mixedγ+ neutron radiation is characteristic of 252 Cf with a half-life of 2.64 years. They are used for contact irradiation, taking into account the neutron component, in the treatment of highly resistant tumors.

2.2. CLINICAL DOSIMETRY

Clinical dosimetry- section of dosimetry of ionizing radiation, which is an integral part of radiation therapy. The main task of clinical dosimetry is the selection and justification of irradiation means that provide the optimal spatiotemporal distribution of absorbed radiation energy in the body of the irradiated patient and a quantitative description of this distribution.

Clinical dosimetry uses computational and experimental techniques. Calculation methods are based on already known physical laws of interaction of various types of radiation with matter. Using experimental methods, treatment situations are simulated with measurements in tissue-equivalent phantoms.

The objectives of clinical dosimetry are:

Measurement of radiation characteristics of therapeutic radiation beams;

Measurement of radiation fields and absorbed doses in phantoms;

Direct measurements of radiation fields and absorbed doses on patients;

Measurement of radiation fields of scattered radiation in canyons with therapeutic installations (for the purpose of radiation safety of patients and personnel);

Carrying out absolute calibration of detectors for clinical dosimetry;

Conducting experimental studies of new therapeutic irradiation techniques.

The basic concepts and quantities of clinical dosimetry are absorbed dose, dose field, dosimetric phantom, target.

Ionizing radiation dose: 1) a measure of radiation received by an irradiated object, the absorbed dose of ionizing radiation;

2) quantitative characteristics of the radiation field - exposure dose and kerma.

Absorbed dose is the basic dosimetric quantity, which is equal to the ratio of the average energy transferred by ionizing radiation to a substance in an elementary volume to the mass of the substance in this volume:

where D is the absorbed dose,

E - average radiation energy,

m is the mass of a substance per unit volume.

The SI unit of absorbed radiation dose is the Gray (Gy) in honor of the English scientist L. N. Gray, known for his work in the field of radiation dosimetry. 1 Gy is equal to the absorbed dose of ionizing radiation, at which the energy of ionizing radiation equal to 1 J is transferred to a substance weighing 1 kg. In practice, an extra-systemic unit of absorbed dose - rad (radiation absorbed dose) is also common. 1 rad = 10 2 J/kg = 100 erg/g = 10 2 Gy or 1 Gy = 100 rad.

The absorbed dose depends on the type, intensity of radiation, its energy and qualitative composition, irradiation time, as well as on the composition of the substance. The longer the radiation time, the greater the dose of ionizing radiation. The dose increment per unit time is called dose rate, which characterizes the rate of accumulation of the dose of ionizing radiation. The use of various special units is allowed (for example, Gy/h, Gy/min, Gy/s, etc.).

The dose of photon radiation (X-ray and gamma radiation) depends on the atomic number of the elements that make up the substance. Under the same irradiation conditions, it is usually higher in heavy substances than in light substances. For example, in the same X-ray field, the absorbed dose in bones is greater than in soft tissues.

In the field of neutron radiation, the main factor determining the formation of the absorbed dose is the nuclear composition of the substance, and not the atomic number of the elements that make up the biological tissue. For soft tissues, the absorbed dose of neutron radiation is largely determined by the interaction of neutrons with the nuclei of carbon, hydrogen, oxygen and nitrogen. The absorbed dose in a biological substance depends on the neutron energy, since neutrons of different energies selectively interact with the nuclei of the substance. In this case, charged particles, γ-radiation can appear, and radioactive nuclei can form, which themselves become sources of ionizing radiation.

Thus, the absorbed dose during neutron irradiation is formed due to the energy of secondary ionizing particles of various natures resulting from the interaction of neutrons with matter.

Absorption of radiation energy causes processes leading to various radiobiological effects. For a specific type of radiation, the output of radiation-induced effects in a certain way

is related to the absorbed radiation energy, often a simple proportional relationship. This allows the radiation dose to be taken as a quantitative measure of the effects of radiation, in particular on a living organism.

Different types of ionizing radiation at the same absorbed dose have different biological effects on the tissues of a living organism, which is determined by their relative biological effectiveness - RBE.

The RBE of radiation depends mainly on differences in the spatial distribution of ionization events caused by corpuscular and electromagnetic radiation in the irradiated substance. The energy transferred by a charged particle per unit length of its path in matter is called linear energy transfer (LET). There are rare ionizing (LET)< 10 кэВ/мкм) и плотноионизирующие (ЛПЭ >10 keV/µm) types of radiation.

Biological effects that arise from different types of ionizing radiation are usually compared with similar effects that occur in an X-ray field with a boundary photon energy of 200 keV, which is taken as exemplary.

RBE coefficient determines the ratio of the absorbed dose of standard radiation that causes a certain biological effect to the absorbed dose of a given radiation that gives the same effect.

where D x is the dose of a given type of radiation for which the RBE is determined, D R is the dose of standard X-ray radiation.

Based on RBE data, different types of ionizing radiation are characterized by their radiative emissivity.

Radiation weighting factor (radiative emissivity)- dimensionless coefficient by which the absorbed dose of radiation in an organ or tissue must be multiplied for calculation equivalent dose radiation to take into account the effectiveness of different types of radiation. The concept of equivalent dose is used to evaluate the biological effect of radiation, regardless of the type of radiation, which is necessary for the purposes of radiation protection of personnel working with sources of ionizing radiation, as well as patients during radiological research and treatment.

Equivalent dose is defined as the average absorbed dose in an organ or tissue, taking into account the average radiation weighting factor.

where H is the equivalent absorbed dose,

W R is the radiation weighting factor currently established by radiation safety standards.

The SI unit of equivalent dose is Sievert (Sv)- named after the Swedish scientist R. M. Sievert, the first chairman of the International Commission on Radiological Protection (ICRP). If in the last formula the absorbed radiation dose (D) is expressed in Grays, then the equivalent dose will be expressed in Sieverts. 1 Sv is equal to the equivalent dose at which the product of the absorbed dose (D) in living tissue of standard composition and the average radiation coefficient (W R) is equal to 1 J/kg.

In practice, a non-systemic unit of equivalent dose is also common - rem(1 Sv = 100 rem), if in the same formula the absorbed dose of radiation is expressed in rads.

Weighting factors for individual types of radiation when calculating equivalent dose.

Effective equivalent dose- a concept used for dosimetric assessment of exposure to healthy organs and tissues and the likelihood of long-term effects. This dose is equal to the sum of the products of the equivalent dose in an organ or tissue by the corresponding weighting factor (weighting factor) for the most important human organs:

where E is the effective equivalent dose,

N T - equivalent dose in organ or tissue T,

W T - weighting factor for organ or tissue T.

The SI unit of effective dose equivalent is the sievert (Sv).

For dosimetric characteristics of the field of photon-ionizing radiation, it is used exposure dose. It is a measure of the ionizing ability of photon radiation in air. SI unit of exposure dose - Pendant per kilogram (C/kg). An exposure dose equal to 1 C/kg means that charged particles released into 1 kg of atmospheric air during the primary acts of absorption and scattering of photons,

When they fully utilize their range in the air, they form ions with a total charge of the same sign equal to 1 Coulomb.

In practice, a non-systemic exposure dose unit is often used X-ray (R)- named after the German physicist Roentgen (W. K. Rontgen): 1 P = 2.58 x10 -4 C/kg.

Exposure dose is used to characterize the field of only photon-ionizing radiation in air. It gives an idea of ​​the potential level of human exposure to ionizing radiation. At an exposure dose of 1 R, the absorbed dose in soft tissue in the same radiation field is approximately 1 rad.

Knowing the exposure dose, it is possible to calculate the absorbed dose and its distribution in any complex object placed in a given radiation field, in particular in the human body. This allows you to plan and control a given irradiation regime.

Currently, more often used as a dosimetric quantity characterizing the radiation field kerma(KERMA is an abbreviation of the expression: Kinetic Energy Released in Material). Kerma is the kinetic energy of all charged particles released by ionizing radiation of any kind, per unit mass of the irradiated substance during the primary acts of interaction of radiation with this substance. Under certain conditions, kerma is equal to the absorbed radiation dose. For photon radiation in air, it is the energy equivalent of the exposure dose. The dimension of kerma is the same as the absorbed dose, expressed in J/kg.

Thus, the concept of “exposure dose” is necessary for assessing the level of dose generated by a radiation source, as well as monitoring the irradiation regime. The concept of “absorbed dose” is used when planning radiation therapy in order to achieve the desired effect (Table 2.1).

Dose field- this is the spatial distribution of the absorbed dose (or its power) in the irradiated part of the patient’s body, a tissue-equivalent environment or a dosimetric phantom that models the patient’s body according to the physical effects of the interaction of radiation with matter, the shape and size of organs and tissues and their anatomical relationships. Information about the dose field is presented in the form of curves connecting points of equal values ​​(absolute or relative) of the absorbed dose. Such curves are called isodoses, and their families - with isodose maps. The absorbed dose at any point in the dose field can be taken as a conventional unit (or 100%), in particular the maximum absorbed dose, which must correspond to the target to be irradiated (that is, the area covering the clinically detected tumor and the expected zone of its spread).

The physical characteristics of the irradiation field are characterized by various parameters. The number of particles that penetrate the medium is called fluence. The sum of all penetrating particles and particles scattered in a given medium is flow ionizing particles, and the flux to area ratio is flux density. Under radiation intensity, or flux density

Table 2.1. Basic radiation quantities and their units

energy, understand the ratio of energy flow to the area of ​​an object. The radiation intensity depends on the particle flux density. Except linear energy transfer (LET), characterizing the average energy losses of particles (photons), determine the linear ionization density (IID), the number of pairs of ions per unit length of path (track) of a particle or photon.

The formation of the dose field depends on the type and source of radiation. When forming a dose field for photon radiation, it is taken into account that the intensity of photon radiation from a point source falls in the medium in inverse proportion to the square of the distance to the source. In dosimetric planning, the concept of average ionization energy is used, which includes the energy of direct ionization and the excitation energy of atoms, leading to secondary radiation, which also causes ionization. For photon radiation, the average ionization energy is equal to the average ionization energy of electrons released by photons.

The dose distribution of the γ-radiation beam is uneven. The 100% isodose region has a relatively small width, and then the relative dose falls along the curve quite steeply. The size of the irradiation field is determined by the width of 50% of the dose. When a dose field of bremsstrahlung radiation is formed, there is a steep dose decline at the field boundary, determined by the small size of the focal spot. This leads to the fact that the width of the 100% isodose is close to the width of the 50% isodose, which determines the dosimetric value of the size of the irradiation field. Thus, in the formation of the dose distribution when irradiated with a bremsstrahlung radiation beam there are advantages over a γ-radiation beam, since the irradiation doses to healthy organs and tissues near the pathological focus are reduced (Table 2.2).

Table 2.2. 100%, 80% and 50% isodose depths at most commonly used radiation energies

Note. The source-surface distance for the X-ray therapy device is 50 cm; gamma therapeutic - 80 cm; linear accelerators - 100 cm.

From the data in table. 2.2 it can be seen that megavolt radiation, unlike orthovoltage X-ray radiation, has a dose maximum not on the surface of the skin; its depth increases with increasing radiation energy (Fig. 13). After the electrons reach their maximum, a steep dose gradient is observed, which makes it possible to reduce the dose load on the underlying healthy tissues.

Protons are distinguished by the absence of radiation scattering in the body and the ability to decelerate the beam at a given depth. In this case, with the penetration depth, the linear energy density (LED) increases, the absorbed dose increases, reaching a maximum at the end of the particle path,

Rice. 13. Distribution of energy of different types of radiation in a tissue-equivalent phantom: 1 - with close-focus X-ray therapy 40 kV and deep X-ray therapy 200 kV; 2 - with gamma therapy 1.25 MeV; 3 - with bremsstrahlung radiation of 25 MeV; 4 - when irradiated with fast electrons 17 MeV; 5 - when irradiated with protons of 190 MeV; 6 - when irradiated with slow neutrons 100 keV

Fig. 14. Bragg Peak

Rice. 15. Gamma radiation dose distribution from two open parallel opposing fields

the so-called Bragg peak, where the dose can be much greater than at the beam entrance, with a steep dose gradient behind the Bragg peak wave to almost 0 (Fig. 14).

Often, during irradiation, parallel opposing fields are used (Fig. 15, see Fig. 16 on the color insert). With a relatively central location of the focus, the dose from each field is usually the same; if the target location area is eccentric, change the dose ratio in favor of the field closest to the tumor, for example 2:1, 3:1, etc.

In cases where the dose is delivered from two non-parallel fields, the smaller the angle between their central axes, the more equalization of isodoses is carried out using the clin.

new filters that make it possible to homogenize the dose distribution (see Fig. 17 on the color plate). To treat deep-lying tumors, three- and four-field irradiation techniques are usually used (Fig. 18).

At a linear electron accelerator, a rectangular radiation field of various sizes is formed using metallic rings.

Rice. 18. Gamma radiation dose distribution from three fields

limators built into the device. Additional beam shaping is achieved using a combination of these collimators and special blocks (a set of lead or Wood's alloy blocks of various shapes and sizes) attached to the LUE after the collimators. The blocks cover parts of the rectangular field outside the target volume and protect tissue beyond the target boundaries, thus forming fields of complex configuration.

The latest linear accelerators allow control over the positions and movements of field-forming multileaf collimators. Typical multileaf collimators have 20 to 80 or more leaves arranged in pairs. Computer control of the position of a large number of narrow petals tightly adjacent to each other makes it possible to generate a field of the required shape. By placing the petals in the required position, a field is obtained that best matches the shape of the tumor. Field adjustments are made through changes to a computer file containing the settings for the petals.

When planning the dose, it is taken into account that the maximum dose (95-107%) must be delivered to the planned target volume, with ≥ 95% of this volume receiving ≥ 95% of the planned dose. Another necessary condition is that only 5% of the volume of organs at risk can receive ≥ 60% of the planned dose.

Typically, linear accelerators have a dosimeter, the detector of which is built into the device for forming the primary beam of bremsstrahlung radiation, that is, the supplied radiation dose is monitored. The dose monitor is often dose calibrated at a reference point located at the depth of maximum ionization.

Dosimetric provision of intracavitary γ-therapy sources high activity designed for individual formation of dose distributions, taking into account the location, extent of the primary tumor, and linear dimensions of the cavity. When planning, calculated data in the form of an atlas of multiplanar isodose distributions attached to intracavitary γ-therapeutic devices, as well as data from planning systems for intracavitary devices based on personal computers can be used.

The presence of a computer planning system for contact therapy allows for clinical and dosimetric analysis for each specific situation with the choice of dose distribution that most fully corresponds to the shape and extent of the primary lesion, which allows reducing the intensity of radiation exposure to surrounding organs.

Before using radiation sources for contact radiation therapy, preliminary dosimetric certification is carried out, for which clinical dosimeters and sets of tissue-equivalent phantoms are used.

For phantom measurements of dose fields, clinical dosimeters with small-sized ionization chambers or other (semiconductor, thermoluminescent) detectors and analyzers are used.

dose field or isodosegraphs. Thermoluminescent detectors (TLDs) are also used to monitor absorbed doses in patients.

Dosimetric devices. Dosimetric instruments can be used to measure doses of one type of radiation or mixed radiation. Radiometers measure the activity or concentration of radioactive substances.

In the detector of a dosimetric device, radiation energy is absorbed, leading to the occurrence of radiation effects, the magnitude of which is measured using measuring devices. In relation to the measuring equipment, the detector is a signal sensor. The readings of the dosimetric device are recorded by an output device (pointer instruments, recorders, electromechanical counters, sound or light alarms, etc.).

Based on the method of operation, dosimetric devices are divided into stationary, portable (can only be carried when switched off) and wearable. A dosimetric device for measuring the radiation dose received by each person in the irradiation zone is called an individual dosimeter.

Depending on the type of detector, there are ionization dosimeters, scintillation dosimeters, luminescent dosimeters, semiconductor dosimeters, photodosimeters, etc.

Ionization chamber is a device for studying and recording nuclear particles and radiation. Its action is based on the ability of fast charged particles to cause ionization of gas. The ionization chamber is an air or gas electric capacitor, to the electrodes of which a potential difference is applied. When ionizing particles enter the space between the electrodes, electrons and gas ions are formed there, which, moving in an electric field, are collected on the electrodes and recorded by recording equipment. Distinguish current And pulse ionization chambers. In current ionization chambers, a galvanometer measures the current created by electrons and ions. Current ionization chambers provide information about the total number of ions formed within 1 s. They are commonly used to measure radiation intensity and for dosimetry measurements.

In pulsed ionization chambers, voltage pulses that occur across the resistance when an ionization current flows through it, caused by the passage of each particle, are recorded and measured.

In ionization chambers for studying γ-radiation, ionization is caused by secondary electrons knocked out from gas atoms or the walls of the ionization chambers. The larger the volume of the ionization chambers, the more ions are formed by secondary electrons, therefore, large-volume ionization chambers are used to measure low-intensity γ-radiation.

The ionization chamber can also be used to measure neutrons. In this case, ionization is caused by recoil nuclei (usually proto-

us), created by fast neutrons, or α-particles, protons or γ-quanta arising from the capture of slow neutrons by nuclei 10 B, 3 He, 113 Cd. These substances are introduced into the gas or the walls of the ionization chambers.

In ionization chambers, the composition of the gas and wall substances is chosen in such a way that, under identical irradiation conditions, the same energy absorption (per unit mass) is ensured in the chamber and biological tissue. In dosimetric instruments, chambers are filled with air to measure exposure doses. An example of an ionization dosimeter is the MRM-2 micro-roentgen meter, which provides a measurement range from 0.01 to 30 µR/s for radiation with photon energies from 25 keV to 3 MeV. Readings are taken using a dial gauge.

IN scintillation In dosimetric instruments, light flashes arising in the scintillator under the influence of radiation are converted using a photomultiplier into electrical signals, which are then recorded by a measuring device. Scintillation dosimeters are most often used in radiation protection dosimetry.

IN luminescent dosimetric instruments use the fact that phosphors are capable of accumulating absorbed radiation energy and then releasing it by luminescence under the influence of additional excitation, which is carried out either by heating the phosphor or by irradiating it. The intensity of the luminescent light flash, measured using special devices, is proportional to the radiation dose. Depending on the luminescence mechanism and the method of additional excitation, there are thermoluminescent (TLD) And radiophotoluminescent dosimeters. A special feature of luminescent dosimeters is the ability to store dose information.

The next stage in the development of luminescent dosimeters was dosimetric devices based on thermal exoelectron emission. When some phosphors, previously irradiated with ionizing radiation, are heated, electrons (exoelectrons) fly out from their surface. Their number is proportional to the radiation dose in the phosphor substance. Thermoluminescent dosimeters are most widely used in clinical dosimetry to measure dose to a patient, in a body cavity, and also as individual dosimeters.

Semiconductor(crystalline) dosimeters change conductivity depending on the dose rate. Widely used along with ionization dosimeters.

In Russia there is a radiation metrological service that carries out verification of clinical dosimeters and dosimetric certification of radiation devices.

At the stage of dosimetric planning, taking into account the data of the topometric map and the clinical task, the physical engineer assesses the dose distribution. The dose distribution obtained in the form of a set of isolines (isodoses) is plotted on a topometric map, and it serves to determine such irradiation parameters as the size of the irradiation field, the location of the centering point of the axes of the radiation beams and their directions.

The single absorbed dose and the total absorbed dose are determined, and the irradiation time is calculated. The document is a protocol containing all the parameters of irradiation of a particular patient at the selected therapeutic installation.

When conducting brachytherapy, the device is used in conjunction with appropriate ultrasound equipment, which makes it possible to evaluate the position of sources and isodose distribution in the organ in a real-time system thanks to the planning system. Another option is to inject sources into the tumor under CT scan guidance.

A radiation beam of the required shape and certain dimensions is formed using an adjustable diaphragm, a collimating device, replaceable standard and individual protective blocks, wedge-shaped and compensating filters and boluses. They make it possible to limit the area and field of irradiation, increase the dose gradient at its boundaries, level out the distribution of the dose of ionizing radiation within the field or, on the contrary, distribute it with the necessary unevenness, create areas and fields, including figured and multiconnected ones (with internal shielded areas).

To correctly reproduce and control the patient’s individual irradiation program, beam visualization devices, mechanical, optical and laser centralizers, standard and individual clamps for immobilizing the patient during irradiation, as well as X-ray and other introscopy tools are used. They are partially built into the radiation head, patient table and other parts of the device. Laser centralizers are mounted on the walls of the treatment room. X-ray introscopes are placed near the therapeutic beam on a floor or ceiling stand with clamps for adjustment in the required position of the patient.

In this chapter we will consider the basic properties of ionizing radiation used in medicine and discuss the processes of their interaction with matter.

Types of ionizing radiation

Let's start by defining some concepts.

Alpha radiation - corpuscular radiation consisting of alpha particles (4 He nuclei) emitted during the radioactive decay of nuclei or during nuclear reactions.Annihilation radiation - photon radiation resulting from the annihilation of a particle and an antiparticle (for example, during the interaction of a p-electron and /? +-positron).

Beta radiation - corpuscular radiation with a continuous energy spectrum, consisting of negatively charged electrons (p-particles) or positively charged positrons (p*-particles) and arising during the radioactive P-decay of nuclei or unstable elementary particles. It is characterized by the limiting (maximum) energy of electrons (positrons).Gamma radiation - photon radiation arising during nuclear transformations or annihilation of particles (energy range from tens of keV to several MeV).

Ionizing radiation" (radiation) - a type of radiation that changes the physical state of atoms or atomic nuclei, turning them into electrically charged ions or products of nuclear reactions (visible light and ultraviolet radiation are not classified as ionizing radiation).

Corpuscular radiation - ionizing radiation consisting of particles with a mass different from zero(a-,fi-particles, neutrons, etc.).

Indirectly ionizing radiation - ionizing radiation, consisting of uncharged particles that can directly create ionizing radiation and (or) cause nuclear transformations (indirectly ionizing radiation can consist of neutrons, photons, etc.).

Neutron radiation - a flow of neutrons that convert their energy in elastic and inelastic interactions with atomic nuclei.

Proton radiation - radiation generated during the spontaneous decay of neutron-deficient atomic nuclei or as a beam at the output of an ion accelerator (for example, a synchrophasotron).

X-ray radiation - photon radiation, consisting of bremsstrahlung and (or) characteristic radiation, generated, for example, by X-ray tubes. Occupies the spectral region between gamma and ultraviolet radiation within the wavelengths mg3+ω0 nm (ω.2 +ω-5 cm). Energy range 100 eV-10.1 MeV. X-rays with a wavelength of less than 0.2 nm (E>50keV) are called hard, with a wavelength of more than about.2 nm (E

Synchrotron (or magnetic bremsstrahlung)radiation - electromagnetic radiation emitted by charged particles moving along trajectories curved by a magnetic field at relativistic speeds.Bremsstrahlung - electromagnetic radiation emitted by a charged particle during its scattering (braking) in an electric field is characterized by a continuous energy spectrum. Sometimes the concept of bremsstrahlung also includes the radiation of relativistic charged particles moving in macroscopic magnetic fields (synchrotron radiation).

Photon radiation - electromagnetic indirectly ionizing radiation that occurs when the energy state of atomic nuclei changes or when particles annihilate.

Characteristic radiation - photon radiation with a discrete energy spectrum that occurs when the energy state of the electrons of an atom changes.

Table 1. Properties of some types of corpuscular radiation.

Ionizing radiation includes photons of electromagnetic radiation (y- and x-rays with a wavelength of less than 20 nm) and corpuscular radiation. Photon radiation with energies between 50 eV and 500 eV is called x-rays, and at higher energies - gamma radiation. Ionizing electromagnetic radiation can be y-radiation accompanying p-decay or arising from the annihilation of positrons, or it can be X-ray bremsstrahlung or characteristic radiation.

Electromagnetic radiation - a disturbance of the electromagnetic field propagating in space (i.e., electric and magnetic fields interacting with each other).

Electromagnetic radiation is a combination of electric and magnetic fields that vary sinusoidally in space and time. Wave speed, And[m/s], is related to the wavelength, L [m], and the oscillation frequency, v: And- L-v, and since And is usually constant, then v=c/A, c=s-th 8 m/s is the speed of light.

Electromagnetic radiation energy (eV):

Where h= 6.626-10-34 Js = 4.135 Yu, 5 eVs.

Electromagnetic radiation has a wide range of energies and various sources: y-radiation of atomic nuclei and bremsstrahlung of accelerated electrons, radio waves, etc. (Table 1, Fig. l). On the scale of electromagnetic waves, y-radiation borders on hard x-ray radiation, occupying the region of higher frequencies. It occurs during the decay of radioactive nuclei and elementary particles, the interaction of fast charged particles with matter, the annihilation of electron-positron pairs, etc. Gamma radiation has a short wavelength (Leu nm) and pronounced corpuscular properties, i.e. behaves like a flow of particles (y-quanta, or photons) with energy /iv.

Bremsstrahlung radiation, which occurs when accelerated electrons pass through a medium, is widely used in medicine. Depending on the energy of the resulting electromagnetic radiation, it is classified as X-ray radiation (energies of tens and hundreds of keV) or y-radiation (energies of one or tens of MeV, but at accelerators they reach energies of several tens of GeV). X-ray radiation is usually obtained using X-ray tubes.

The intensity of bremsstrahlung is proportional to the square of the acceleration of the charged particle. Since acceleration is inversely proportional to the mass of the particle, in the same field the bremsstrahlung radiation of an electron is millions of times more powerful than the radiation of a proton. Therefore, bremsstrahlung radiation, which occurs when electrons are scattered in the electrostatic field of atomic nuclei and electrons, is most often used.

Rice. 1.

The spectrum of bremsstrahlung photons is continuous and ends at the maximum possible energy, equal to the initial energy of the electron. Since the intensity of bremsstrahlung radiation is proportional to Z 2 , to increase the yield of bremsstrahlung photons in electron beams, targets made of substances with large Z are used.

Corpuscular ionizing radiation includes a-radiation, electron, proton, neutron and meson radiation. Corpuscular radiation, consisting of a stream of charged particles (a-, (3-particles, protons, electrons), the kinetic energy of which is sufficient to ionize atoms upon collision with them, belongs to the class of directly ionizing radiation. Neutrons themselves do not produce ionization, but in the process interactions with the environment release charged particles (electrons, protons) capable of ionizing atoms and molecules of the medium through which they pass. Neutron radiation is classified as indirectly ionizing radiation.

Neutrons vary significantly in their energies. To describe the energy characteristics of neutron radiation, the concept of neutron spectrum is used. Neutrons are classified by their speed of movement:

• - Relativistic neutrons, with energy more than 10 eV;
• - Fast neutrons, with energy greater than o.i MeV (sometimes greater than i MeV)
• - Slow neutrons - neutrons with energy less than 100 KeV. or by “temperature”:
• - Epithermal neutrons, with energy from 0.025 to 1 eV;
• - Hot neutrons, with an energy of about 0.2 eV;
• - Thermal neutrons, with an energy of approximately 0.025 eV;
• - Cold neutrons, with energy from 510-5 eV to 0.025 eV;
• - Very cold neutrons, with energy 2*10-? - 5*10-5 eV;
• - Ultracold neutrons, with energy less than 2*10-? eV.

The interaction of neutrons with atoms is weak, which allows neutrons to penetrate deeply into matter.

Electron radiation is usually a beam of electrons at the output of an electron accelerator. It is characterized by the average radiation energy and dispersion (scatter), as well as the beam width. Special measures can be used to obtain a monoenergetic narrow beam of high-energy electrons.

Beta radiation accompanies the most common type of radioactive decay of nuclei - p-decay. Since the speed of p-particles is much higher than the speed of a-particles, they interact less frequently with atoms of the medium; Their ionization density per unit path is hundreds of times lower than that of alpha particles, and the path in air reaches 10 m. In biological soft tissue, the path is equal to 10+12 mm. This radiation is absorbed by a 1 mm thick layer of aluminum. Unlike electron radiation, p-radiation is accompanied by a flux of antineutrinos for electrons and neutrinos for positrons. Positron radiation is also accompanied by annihilation y-radiation (with an energy of 0.51 and/or 1.02 MeV).

The first studies of ionizing radiation were carried out at the end of the 19th century. In 1895, German physicist W.K. Roentgen discovered “X-rays,” later called X-rays. In 1896, the French physicist A. Becquerel discovered traces of natural radioactivity of uranium salts on photographic plates. In 1898, the spouses Marie and Pierre Curie discovered that uranium, after radiation, turns into other chemical elements. They named one of these elements “radium” (Ra) (from the Latin “emitting rays”).

Ionizing radiation is radiation whose interaction with a medium leads to the formation of ions of different signs. Ionizing radiation is divided into corpuscular and photon.

Corpuscular radiation includes: a, b-, proton and neutron radiation.

a-radiation is a stream of helium nuclei formed during radioactive decay. They have a mass of 4 and a charge of +2. The a-emitters include about 160 natural and man-made radionuclides, most of which are at the end of the periodic table of elements (nuclear charge > 82). a-particles propagate rectilinearly in media and have a small range (the distance at which particles lose their energy when interacting with matter): in air - less than 10 cm; in biological tissues 30-150 microns. a - particles have high ionizing and low penetrating ability.

b-radiation is a flow of electrons and positrons. Their mass is tens of thousands of times less than the mass of a-particles. b-emitters include about 690 natural and man-made emitters. The range of b-particles in the air is several meters, and in biological tissues - about 1 cm. They have a higher penetrating ability than a-particles, but less ionizing power.

Proton radiation– flow of hydrogen nuclei.

Neutron radiation– a flow of nuclear particles that do not have a charge with a mass close to the mass of a proton. Free neutrons are captured by nuclei. In this case, the nuclei go into an excited state and fission with the release of g-quanta, neutrons and delayed neutrons. Thanks to delayed neutrons, the fission reaction in nuclear reactors is controlled. Neutron radiation has a higher ionizing ability compared to other types of corpuscular radiation.

Photon is a quantum of energy of high-frequency electromagnetic radiation. Photon radiation is divided into x-rays and gamma radiation. They have high penetrating and low ionizing ability.

X-ray radiation- This is artificial electromagnetic radiation that occurs in X-ray tubes (“X-rays”).

g-radiation – This is electromagnetic radiation of natural origin. g-rays propagate rectilinearly, do not deviate in electric and magnetic fields, and have a long range in the air.

Directly ionizing radiation– this is radiation consisting of charged particles, for example, a, b-particles. Indirectly ionizing radiation is radiation consisting of uncharged particles, such as neutrons or photons. They create secondary radiation in the environments through which they pass.

Ionizing radiation is described by the following physical quantities

Activity of substance A determined by the rate of radioactive decay:

where: dN – number of spontaneous nuclear transformations during time dt.

Activity units:

in the SI system - Becquerel: 1 Bq = 1 dispersion/s

extra-systemic unit – Curie: 1 Ci = 3.7. 10 10 dispersion/s, which corresponds to the activity of 1 g of pure Ra.

Half-life T 1/2– time required to reduce the activity of radionuclides by 2 times. For U-238 T 1/2 = 4.56. 10 9 years, for Ra-226 T 1/2 = 1622 years.

Exposure dose X– the energy of ionizing radiation causing the formation in the air of a charge dQ of the same sign in an elementary volume with a mass dm.

Exposure dose units:

in the SI system 1 C/kg = 3880 R.

non-systemic unit – X-ray: 1 R

The absorbed dose D is determined by the amount of absorbed energy dE per unit mass of the irradiated substance dm.

Absorbed dose units:

in SI Gray: 1 Gy

off-system unit 1 rad = 0.01 Gy

1 Р = 0.87 rad

1 rad = 1.14 R

The name “rad” comes from the first letters of the term “radiation absorbed dose”.

Equivalent dose H R shows the danger of various types of radiation exposure of biological tissues and is equal to:

where: W R is a weighting coefficient reflecting the danger of a particular type of ionizing radiation for the body.

X-ray, g-radiation, b-radiation W R = 1;

neutrons W R = 5-20;

a-particles W R = 20.

Equivalent dose units:

in the SI system 1 Sv in honor of the Swedish scientist Sievert

off-system unit – 1 rem = 0.01 Sv

rem is the biological equivalent of rad.

Effective equivalent dose H E– this is the magnitude of the risk of long-term consequences of irradiation of the entire human body and its individual organs, taking into account their radiosensitivity. Different organs and tissues have different sensitivity to radiation. For example, with the same equivalent dose of H R radiation, lung cancer is more likely to occur than thyroid cancer. Therefore, the concept of effective equivalent dose was introduced.

where: W T – weighting coefficient for biological tissue.

Photon IR includes g-radiation of radioactive substances, X-ray characteristic and bremsstrahlung radiation generated by various accelerators. The ABI of photon radiation is the lowest (1-2 pairs of ions per 1 cm 3 of air), which determines its high penetrating ability (in air the path length is several hundred meters).

g-radiation occurs during radioactive decay. The transition of a nucleus from an excited to a ground state is accompanied by the emission of a gamma quantum with energies from 10 keV to 5 MeV. The main therapeutic sources of g-radiation are remote gamma-therapeutic devices with the artificial radionuclide 60 Co. This artificial radioactive emitter has been used in radiotherapy clinics for over 60 years due to its characteristics. The energy of 60 Co gamma radiation is quite high and amounts to 1.25 MeV, which allows the beam energy to move deep into the tissue. With a maximum relative absorbed dose at a depth of 0.5 cm, 50% of the depth dose is located at a depth of 11.4 cm. A fairly long half-life of 5.3 years, due to which the power of the source decreases over a long period of time, and recharging of the device is required once at 5-7 years old.

High-energy bremsstrahlung X-rays arises due to the acceleration and sharp deceleration of accelerated high-energy electrons in the vacuum systems of various accelerators and differs from X-ray by a higher quantum energy (from one to tens of MeV).

When a stream of photons passes through a substance, it is weakened as a result of the interaction of ionizing radiation with the substance. The type of interaction of photons with atoms of matter depends on the energy of the photons. The following types of interaction of photons with matter are distinguished:

· Classical (coherent, or Thompson, scattering) - for photons with energy from 10 to 50-100 keV. The relative frequency of this effect is small. An interaction occurs, which does not play a significant role, since the incident quantum, colliding with an electron, is deflected and its energy does not change.

· Photoelectric absorption (photoelectric effect) - at relatively low energies - from 50 to 300 keV (plays a significant role in x-ray therapy). The incident quantum knocks out the orbital electron from the atom, is itself absorbed, and the electron, slightly changing direction, flies away. This escaped electron is called a photoelectron. Thus, the energy of the photon is spent on the work function of the electron and on giving it kinetic energy. An atom that has lost an electron turns into a positive ion, and the photoelectron at the end of its run loses its energy and joins a neutral atom, turning it into a negatively charged ion.

· Compton effect (incoherent scattering) - occurs at photon energies from 120 keV to 20 MeV (i.e., almost the entire spectrum of energies used in radiation therapy). An incident quantum knocks out an electron from the outer shell of an atom, transferring part of the energy to it, the remaining part changes its direction. The electron flies out of the atom at a certain angle, and the new quantum differs from the original one not only in a different direction of movement, but also in lower energy. The resulting quantum will indirectly ionize the medium, and the electron will ionize directly.

· The process of formation of electron-positron pairs - the quantum energy must be greater than 1.02 MeV (twice the rest energy of the electron). This mechanism has to be taken into account when patients are irradiated with high-energy bremsstrahlung radiation. Near the nucleus of an atom, the incident quantum experiences acceleration and disappears, transforming into an electron and a positron. The positron quickly combines with an oncoming electron, and the process of annihilation (mutual destruction) occurs, and in return two photons appear, the energy of each of which is half the energy of the original photon. Thus, the energy of the primary quantum transforms into the kinetic energy of the electron and into the energy of annihilation radiation.

· Photonuclear absorption - the energy of the quanta must be greater than 2.5 MeV. The photon is absorbed by the nucleus of an atom, as a result of which the nucleus goes into an excited state and can either give up an electron or fall apart. This is how neutrons are produced.

As a result of the above processes of interaction of photon radiation with matter, secondary photon and corpuscular radiation (electrons and positrons) arises. The ionization ability of charged particles is much greater than that of photon radiation.

The spatial attenuation of the photon beam occurs according to an exponential law (inverse square law): The radiation intensity is inversely proportional to the square of the distance to the radiation source.

Radiation in the energy range from 200 keV to 20 MeV has found the widest application in the treatment of malignant neoplasms. Greater penetrating power allows energy to be transferred to deep-lying tumors. At the same time, the radiation exposure to the skin and subcutaneous tissue is sharply reduced, which makes it possible to deliver the required dose to the lesion without radiation damage to these areas of the body (unlike soft X-ray radiation). With an increase in photon energy above 15 MeV, the risk of radiation damage to tissue at the beam output increases.

In general (Fig. 2.3), the penetrating ability of ionizing radiation, and, consequently, the indications for their use in tumor therapy depends on the type of radiation (photon radiation has a generally higher penetrating ability than corpuscular radiation) and its energy (increases).

Rice. 2.3.Examples of the percentage linear dose distribution of electron beams with energy from 6 to 20 MeV and photon radiation from gamma radiation 60Co to megavolt X-ray radiation with energy 4-25 MeV.

Test questions for the section

(correct answers are highlighted)

1) Arrange the components of the decay of the nucleus of a radioactive substance in order of increasing their penetrating ability in tissues

a) Alpha radiation, gamma radiation, beta radiation

b) Gamma radiation, alpha radiation, beta radiation

c) Alpha radiation, beta radiation, gamma radiation

d) The penetrating ability of the components may vary depending on the state of aggregation of the substance

2) What is the penetrating ability of accelerated electrons, characterized by the mean free path in human tissue?

b) does not exceed 2 cm

d) up to 10 cm

3) The main advantages of using proton beams in radiation therapy are:

a) formation of non-diverging beams;

b) synchronization of the beam with breathing

c) the ability to supply the required amount of energy to a given depth corresponding to the Bragg peak;

d) high dose gradient (selectivity) between the target and surrounding tissues;

e) conformality of irradiation

4) What charge do pi mesons have?

a) Negative

b) Positive

c) Double positive

d) Have no charge

5) What charge do alpha particles have?

a) Negative

b) Positive

c) Double positive

d) Have no charge

6) What charge do neutrons have?

a) Negative

b) Positive

c) Double positive

d) Have no charge

7) What localization of malignant tumors is preferable for the use of neutron capture therapy?

a) Muscles and fatty tissue;

b) Brain

c) Tumors of the abdominal organs

d) Localization does not matter

8) The ABI of photon radiation in air is...

a) 1-2 pairs of ions per 1 cm 3

b) 5-10 pairs of ions per 1 cm 3

c) 50-70 pairs of ions per 1 cm 3

d) 200--300 pairs of ions per 1 cm 3

9) In what energy range of photons when interacting with matter is the photoelectric effect observed?

a) 10-20 MeV

c) 50-300 KeV

e) More than 1.02 MeV

e) 120 KeV – 20 MeV

10) In what energy range of photons when interacting with matter is the Compton effect observed?

a) 10-100 keV

c) 50-300 KeV

e) More than 1.02 MeV

e) 120 KeV – 20 MeV

11) In what energy range of photons during interaction with matter is the process of formation of electron-positron pairs observed?

a) 10-20 MeV

c) 50-300 KeV

e) More than 1.02 MeV

e) 120 KeV – 20 MeV

12) In accordance with the “inverse square law”, the radiation intensity is inversely proportional to the square...

a) Distances from the radiation source

b) Initial energy of the photon radiation beam

c) Magnetic induction of the beam

13) What type of ionizing radiation has the highest penetrating ability in biological tissues?

a) X-ray orthovoltage

b) Accelerated electrons

c) Gamma radiation

d) Alpha particles

e) High-energy bremsstrahlung

e) X-ray 50 KeV

14) What type of radiation is produced during the radioactive decay of the radionuclide 60 Co?

a) X-ray

b) Beta radiation

c) Neutrons

d) Gamma radiation

e) Protons

f) Accelerated electrons

15) What is the half-life of the radionuclide 60 Co (cobalt sixty)?

a) 2.3 years

d) 4.8 months

e) 5.2 years

e) 4.5 years

16) What is the energy of gamma radiation from the radionuclide 60 Co?

Photon radiation includes radiation from radioactive substances, characteristic and bremsstrahlung radiation generated by various accelerators. The ABI of photon radiation is the lowest (1-2 pairs of ions per 1 cm 3 of air), which determines its high penetrating ability (in air the path length is several hundred meters).

-radiation occurs during radioactive decay. The transition of a nucleus from an excited to a ground state is accompanied by the emission of a -quantum with energies from 10 keV to 5 MeV. The main therapeutic sources of -radiation are -devices (guns).

Bremsstrahlung X-rays arises due to the acceleration and sharp deceleration of electrons in the vacuum systems of various accelerators and differs from X-ray by a higher quantum energy (from one to tens of MeV).

When a stream of photons passes through a substance, it is weakened as a result of the following interaction processes (the type of interaction of photons with atoms of the substance depends on the energy of the photons):

Classical (coherent, or Thompson, scattering) - for photons with energy from 10 to 50-100 keV. The relative frequency of this effect is small. An interaction occurs, which does not play a significant role, since the incident quantum, colliding with an electron, is deflected and its energy does not change.

Photoelectric absorption (photoelectric effect) - at relatively low energies - from 50 to 300 keV (plays a significant role in x-ray therapy). The incident quantum knocks out the orbital electron from the atom, is itself absorbed, and the electron, slightly changing direction, flies away. This escaped electron is called a photoelectron. Thus, the energy of the photon is spent on the work function of the electron and on giving it kinetic energy.

Compton effect (incoherent scattering) - occurs at photon energies from 120 keV to 20 MeV (i.e., almost the entire spectrum of radiation therapy). The incident quantum knocks out an electron from the outer shell of the atom, transferring part of the energy to it, and changes its direction. The electron flies out of the atom at a certain angle, and the new quantum differs from the original one not only in a different direction of movement, but also in lower energy. The resulting quantum will indirectly ionize the medium, and the electron will ionize directly.

The process of formation of electron-positron pairs - the quantum energy must be greater than 1.02 MeV (twice the rest energy of the electron). This mechanism has to be taken into account when a patient is irradiated with a high-energy bremsstrahlung radiation beam, i.e., at high-energy linear accelerators. Near the nucleus of an atom, the incident quantum experiences acceleration and disappears, transforming into an electron and a positron. The positron quickly combines with an oncoming electron, and the process of annihilation (mutual destruction) occurs, and in return two photons appear, the energy of each of which is half the energy of the original photon. Thus, the energy of the primary quantum transforms into the kinetic energy of the electron and into the energy of annihilation radiation.

Photo nuclear absorption - the energy of the quanta must be greater than 2.5 MeV. The photon is absorbed by the nucleus of an atom, as a result of which the nucleus goes into an excited state and can either give up an electron or fall apart. This is how neutrons are produced.

As a result of the above processes of interaction of photon radiation with matter, secondary photon and corpuscular radiation (electrons and positrons) arises. The ionization ability of particles is much greater than that of photon radiation.

The spatial attenuation of the photon beam occurs according to an exponential law (inverse square law): The radiation intensity is inversely proportional to the square of the distance to the radiation source.

Radiation in the energy range from 200 keV to 15 MeV has found the widest application in the treatment of malignant neoplasms. Greater penetrating power allows energy to be transferred to deep-lying tumors. At the same time, the radiation exposure to the skin and subcutaneous tissue is sharply reduced, which makes it possible to deliver the required dose to the lesion without radiation damage to these areas of the body (unlike soft X-ray radiation). With an increase in photon energy above 15 MeV, the risk of radiation damage to tissue at the beam exit increases.