Radiobiological rationale and patient selection for high-LET radiation in cancer therapy.

The rationale for introducing ion beams in cancer therapy is the high level of physical selectivity that can be achieved with ions, equal or even better than with proton beams or modern photon techniques, as well as the potential advantage of high-LET radiations for some tumour types and sites. The radiobiological arguments for high-LET radiation in cancer therapy are reviewed: reduction of OER in the case of hypoxic and poorly-reoxygenating tumours, and the lesser importance of repair phenomena which are a problem in controlling repair-proficient photon-resistant tumours. Fast neutrons were the first type of high-LET radiation used clinically, and were often applied under suboptimal technical conditions. Nevertheless, useful clinical information was derived from the neutron experience. A greater benefit from neutrons than from conventional radiotherapy was found for several tumour sites. The present discussion is limited to the results for salivary gland tumours and prostatic adenocarcinoma. Based on the fast neutron experience, radiobiological arguments, and the added benefit of excellent physical selectivity of ion beams, the potential clinical indications for high-LET ions are discussed: hypoxic, slowly growing and well-differentiated photon-resistant tumours. One of the main remaining issues is the selection of individual patients for high- or low-LET radiation. Since the physical selectivity of ions now matches that obtained with other techniques, the selection of patients will be based only on the radiobiological characteristics of the tumour.