Proton therapy offers several advantages compared with classical radiotherapy owing to a better dose conformity to the tumour volume. However, proton interactions with beam transport elements and the human tissues lead to the production of secondary neutrons, resulting in an extra whole-body dose with some carcinogenic potential. In this study, the secondary neutron doses generated with an active beam scanning system and with two compact proton therapy systems recently appeared on the market are compared.
With no RS, the proton spots are ∼20-70% larger at isocenter in the UN than in the DN. Spots are less asymmetric, and eccentricity increases more slowly with energy, in the UN than in the DN. Over the 33 cm in-air travel upstream of isocenter, the spot FWHM varies by less than ∼2 mm. However, spot asymmetry becomes more severe upstream (for 115 MeV spots, 30-40% compared to <20% at isocenter for DN, but similar and <10% for UN). With an RS, spot FWHM at isocenter increases by 12.7 mm from 8.3 mm (DN) and 10.7 mm from 13 mm (UN) for 150 MeV protons (typical for brain treatments). With no RS, relatively distance-independent spot size facilitates SAD-type treatments. For patients with superficial lesions, where an RS is required and the phase space varies rapidly with distance, the RS should be permitted at two additional locations. US Army Medical Research and Materiel Command under Contact Agreement No. DAMD17-W81XWH-04-2-0022.
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