Influence of the patient size on secondary doses delivered in light ion beams

*Secondary radiation exposure of patients in light ion therapy is of concern due to possible normal tissue damage and risk of induction of secondary cancers. Neutrons, protons and heavier ions are generated by nuclear inelastic interactions of primary ions both in the beam line and in the patient. The patient is exposed to a complex radiation field and secondary doses can be deposited in normal tissues both close to and relatively far from the treated volume. The energy distribution of secondary particles and secondary doses delivered to different organs were studied by the MC code SHIELD-HIT10 using the anthropomorphic phantoms representing a 10-year-old child (CHILD-HIT) and an adult male (ADAM-HIT). Brain tumor irradiations were simulated with approximated scanned beams of 1 H, 7 Li and 12 C ions. The influence of patient size on the secondary dose distributions were studied using the same target definition and irradiation geometry with a lateral beam. For the scanned beams, the secondary organ absorbed doses normalized per absorbed dose to the treated volume (brain tumor) were in the range 1 nGy/Gy - 0.1 mGy/Gy and the absorbed doses in the CHILD-HIT phantom were higher than in ADAM-HIT by up to a factor of 5, depending on organ.