Assessment of the Possibility of Verification of Proton Dose Distributions by the Method of Induced Positron Activity in Human Tissue

A computational assessment was made of the possibility of verifying dose distributions during proton radiation therapy using PET imaging of positron activity in human tissues, which was formed as a result of proton irradiation. To compare the dose distribution of a particle energy-modulated proton beam with a diameter of 10 mm with an initial particle energy of 100 MeV, ensuring uniform irradiation of the target in a 13 mm zone (at the level of 90 % of the radiation dose) at the end of the particle path, with a map of induced activity from the radionuclides 11C, 13N and 15O, numerical calculations were performed in a Monte–Carlo code using the Geant4 simulation program. In the modeling process, a volume with dimensions of 50 × 50 × 100 mm was used, simulating soft tissues of the human body with a density of 1 g/cm3, consisting of hydrogen atoms (62 %), carbon (12 %), oxygen (24 %) and nitrogen (1.1 %). The cross sections for the formation of radionuclides 11C, 13N and 15O in the reactions 12C(p, pn)11C, 14N(p, α)11C, 16O(p, αpn)11C, 14N(p, pn)13N, 16O(p, α)13N, 16O(p, pn)15O have been calculated, which were used to calculate the distributions of positron activity in the irradiated volume. Taking into account the short half-lives of the radionuclides under consideration (primarily oxygen-15), calculations of isoactivities and depth distributions of accumulated radioactivities were performed for various time intervals after irradiation. The performed computational modeling of the distributions of activities of radionuclides 11C, 13N and 15O during the passage of a modulated proton beam, taking into account the decay of produced radionuclides after irradiation, shows that by recording for 15 minutes the induced activity of PET radionuclides 2 minutes after irradiation, it is possible to obtain data on the compliance of the planned and irradiation of tumors performed during proton therapy. However, small levels of generated activity (at a level of 2 Gy for finely fractionated irradiations) require a device with high efficiency in recording annihilation radiation and high spatial resolution at the level of 1.5–2.0 mm.