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.

Key words: proton irradiation, radionuclides, induced activity, PET, dose fields, verification