Negative pion depth-dose profile examined by means of HeLa cell survival curves

HeLa cells were irradiated at liquid nitrogen temperature with negative pions from Nimrod at the Rutherford Laboratory and then assayed for survival of colony-forming ability. Complete dose-response curves were obtained from repeated determinations at fourteen different positions along the depth dose profile and survival curves fitted to the data by computer programme. A depth damage profile was thus established in terms of the final slopes of these curves. This confirmed the expected RBE value of the 1.9 at the ionisation peak. Although a value of unity was found in the main plateau region, the 'entrance' positions showed significantly higher values. (author)