Biological aspects of high LET radiation therapy

Biological information suggests that high LET radiations could be used advantageously in radiation therapy. The radiation responses of cells and tissues are described in general terms with respect to the following aspects of high LET radiations considered for therapeutic use: spatial distribution of biological effect, oxygen enhancement of cell lethality in mixed LET radiation beams, dependence of oxygen enhancement of cell lethality upon the microscopic distribution of energy loss, gain factors due to relative biological effectiveness alone, and recovery from sublethal cellular damage. It is concluded that there is no single advantage to the use of high LET radiations in radiation therapy, but that combinations of advantages may exist for different radiations and different tumor-normal tissue combinations. Cases in which inverse gain factors occur are to be avoided.