Impact of irradiation setup in proton spot scanning brain therapy on organ doses from secondary radiation

Abstract

A Monte Carlo model of a proton spot scanning pencil beam was used to simulate organ doses from secondary radiation produced from brain tumour treatments delivered with either a lateralfield or a vertexfield to one adult and one paediatric patient. Absorbed doses from secondary neutrons, photons and protons and neutron equivalent doses were higher for the vertexfield in both patients, but the differences were low in absolute terms. Absorbed doses ranged between 0.1 and 43μGy. Gy -1 in both patients with the paediatric patient receiving higher doses. The neutron equivalent doses to the organs ranged between 0.5 and 141μSv. Gy -1 for the paediatric patient and between 0.2 and 134μSv. Gy -1 for the adult. The highest neutron equivalent dose from the entire treatment was 7 mSv regardless of field setup and patient size. The results indicate that differentfield setups do not introduce large absolute variations in out-of-field doses produced in patients undergoing proton pencil beam scanning of centrally located brain tumours.