Prospects for the application of fast neutrons in cancer therapy. Radiobiological bases and survey of the clinical data.

The rationale for introducing fast neutrons in therapy initially was a reduction in the Oxygen Enhancement Ratio (OER). The recent radiobiological developments indicate that, more generally, fast neutrons tend to reduce the difference in radiosensitivity between cell lines, or related to the degree of oxygenation (Oxygen Gain Factor), the position of the cells in the mitotic cycle (Kinetics Gain Factor), etc. The reduction of the differences in radiosensitivity brings a benefit for certain types of tumours (normally resistant to photons), but a disadvantage for other types of tumours (those which can currently be controlled by photon treatment). A review of the available clinical data is presented. The tumour types or sites for which a benefit has been observed are discussed: locally advanced tumours of the salivary glands, paranasal sinuses, some tumours of the head and neck area with metastatic lymph nodes, slowly growing, well differentiated soft tissue sarcomas, inoperable/recurrent melanomas, locally extended (C, D1) prostatic adenocarcinomas. Selection of the patients suitable for neutron therapy remains the main problem. Collaboration between neutron therapy centres is essential to accelerate the acquisition of sufficient clinical data needed in order to improve patient selection, as well as the optimum modality of application of fast neutrons.