Distributions of beta+ decayed nuclei generated in the CH2 and H2O targets by the target nuclear fragment reaction using therapeutic MONO and SOBP proton beam.

In proton radiotherapy, the irradiation dose can be concentrated on a tumor. To use this radiotherapy efficiently in the clinical field, it is necessary to evaluate the proton-irradiated area and condition. The proton-irradiated area can be confirmed by coincidence detection of pair annihilation gamma rays from beta+ decayed nuclei generated by target nuclear fragment reaction of irradiated proton nuclei and nuclei in the irradiation target. In this study, we performed experiments of proton irradiation to a polyethylene (PE:CH2) target containing 12C nuclei, which is a major component of the human body, and a gelatinous water (H2O) target containing 16O nuclei at different proton irradiation energy levels under different beam conditions of mono-energetic Bragg peak and spread-out Bragg peak. The distribution of the activity in the target after proton irradiation was measured by a positron emission tomography (PET) apparatus, and compared with the calculated distribution. The temporal dependence of the activity distribution during the period between the completion of proton irradiation and the start of measurement by the PET apparatus was examined. The activity by clinical proton irradiation was 3 kB/cc in the PE target and 13 kB/cc in the water target, indicating that the intensity was sufficient for the evaluation of the distribution. The range of the activity distribution against the physical range was short (several millimeter water equivalent length), indicating the presence of target dependence. The range difference in the water target was slightly large with time dependence until the start of measurement. The difference of the lateral widths with full width half at maximum in the distributions of the measured irradiated dose and activity was within 1 mm.