Development of a method for control of dose distribution in radiation therapy on photon beams

Today, radiation therapy has a significant place in the treatment of cancer. In recent years, new radiotherapy methods of treatment have begun to be introduced, which, for example, make it possible to control tumors directly during the irradiation process, shortening the procedure time and minimizing the negative impact on the patient. An example of combining diagnostics and radiation therapy is the "Tomotherapy" device, with the help of which a beam of ionizing radiation is directed to the tumor with high accuracy, the built-in computer form determines the size and position of the tumor in a matter of seconds before the start of the session. We propose a method for assessing the distribution of doses in the body during irradiation with medical electron accelerators. The method uses positron emission tomography based on the registration of annihilation photons. It is based on the use of the electron-positron pair production mechanism. The positrons formed in the course of irradiation with photons undergo ionization deceleration and then annihilation with the formation of two photons with an energy of 0.511 MeV. By registering such photons for coincidence, it is possible to obtain the distribution of annihilation events in the irradiated tissue region. In the work, a mathematical model has been created that allows one to estimate the absorbed dose in a given tissue site. Within the framework of the computer experiment performed using the GEANT4 package, the correlation between the distribution of the absorbed dose of the primary photon flux and the distribution of the annihilation density of secondary positrons was estimated. The depth distributions of the absorbed dose and the number of annihilations are obtained, and an empirical recalculation function from the distribution of the number of annihilations in the depth estimate of the absorbed dose is selected.