Differences between biological effects of high LET and low LET radiations in relation to their application in radiotherapy.

The application of fast neutrons, negative pions or heavy ions will only provide an advantage for the radiotherapy of cancer if, in comparison with conventional radiation, e.g., X-rays, gamma rays and electrons, better depth-dose and collimation characteristics or specific radiobiological dose-response relationships result in greater local control probabilities for tumours without increased frequencies of severe normal tissue damage. Differences in intrinsic radiosensitivity and the presence of hypoxic cells are considered to be the main factors which can cause values of the relative biological effectiveness (RBE) for responses of tumours to be larger than RBE values for normal tissue tolerance. Clinical studies on lung metastases irradiated with single doses of 15 MeV neutrons indicate that RBE values for tumour growth delay can vary between 1.2 and 4.0, while RBE values for damage to several normal tissues are estimated from data on a few animal tissues to be approximately 2.5.