A review of high LET facilities, existing and projected, with emphasis on the radiobiologic aspects.

The introduction of high LET radiations into radiotherapy is an exciting development. Neutron facilities are already in clinical use in England and the United States, while machines to produce negative pi mesons and high energy heavy ions are in an advanced stage of development in Canada, the United States and Europe. The state of the art at each of these installations is reviewed. The therapeutic merits of various heavy particles are compared with conventional photon beams. Lowering the OER (oxygen enhancement ratio) and improvement in the depth dose pattern are the principal advantages sought. Neutrons result in a somewhat lower OER but offer depth-dose patterns that are barely the equal of x- and gamma-rays. Protons by contrast produce excellent dose distributions, but no reduction of OER. Negative pi mesons improve both depth dose pattern and OER to some extent. High energy heavy nuclei show the greatest promise as far as lowering the OER is concerned, but will probably not equal pions in depth dose pattern. More radiobiologic data are urgently needed for all of these new radiation modalities.