The RBE of megavoltage photon and electron beams

A recent MRC Working Party (1975) was convened to discuss the relative benefits of electron or photon megavoltage beams for therapy. The discussion arose due to a feeling among many radiotherapists that normal tissues (especially skin) would tolerate higher doses from electron beams (up to 20%) than from photon beams of similar primary energy. This is contrary to physical expectations and to existing biological measurements. It has been suggested that the contradiction may be due to known differences in depth dose distributions and to some tissue inhomogeneities. To clarify the discrepancy between clinical observations on the one hand and biological measurements on the other, further experiments have been performed to measure RBE values for a gross tissue response where the dose distributions have been kept almost identical.