Hadrons (protons, neutrons, heavy ions) in radiation therapy: rationale, achievements and expectations

The safety, efficacy and reliability of photon beams is well established; h owever, new types of beams continue to be introduced and explored with the aim of achieving improved selectivity from better dose distribution or from radiobiological mechanisms. Fast neutrons were the first non-conventional radiation used in cancer therapy. Fast neutrons, a form of high-LET radiati on, were introduced for the following radiobiological reasons: (1) a reduct ion of the OER with increasing LET, (2) a reduction in the difference in ra diosensitivity related to the position of the cells in the mitotic cycle, ( 3) less repair and thus less clinical relevance of the different repair mec hanisms. The best, clinically proven, indications for fast neutrons are sal ivary gland tumours, locally advanced prostatic adenocarcinomas and slowly growing, well differentiated sarcomas. Proton beams brought a significant i mprovement in the physical selectivity. Worldwide, the number of proton the rapy centres in operation and in the planning stage increases continuously. The best clinical results so far have been reported for uveal melanoma, tu mours of the base of skull, and some brain tumours in children. Heavy ions combine the advantage of better physical selectivity of protons with the ra diobiological advantages of fast neutrons for some tumour types. Heavy ions were applied at Berkeley from 1975 to 1992, and at NIRS in Chiba since 199 4. A pilot study started at GSI-Darmstadt in 1997. Boron neutron capture th erapy (BNCT) aims at a physical selectivity at the cellular level; it is st ill in an experimental phase.