Cell survival over the depth profile after irradiation with a negative pion beam.

The colony-forming ability of Chinese hamster fibroblast cells in vitro has been investigated throughout the depth dose profile of a negative pion beam. In the peak and 2 cm beyond, the RBE for macroclones (more than 50 cells per clone) was 1.3 and in the plateau region 0.8--1.0. All of the microclones (2-49 cells per clone) were observed as well. The RBE values are the same for peak and somewhat higher for plateau and post peak than found in cell survival studies referring to macroclones.