Biological effects of negative pions

Most pion radiobiological work in the past was done at low dose rates using biological systems sensitive to small doses. Biological effects at the beam entrance were found to be nearly the same as conventional radiations, although some reports have indicated a higher RBE. LET distribution at the peak of a nearly monoenergetic pion beam is not much different from fast neutrons; therefore, one would expect similar biological responses, and the results are consistent with this hypothesis. The RBE at the peak for a nearly monoenergetic pion beam is significantly higher than at the entrance. The RBE peak/RBE plateau ratio is decreased with increasing width of the peak. The OER (∼1.6) at the peak for a nearly monoenergetic pion beam is also similar to fast neutrons and is expected to increase with increasing width of the peak. No OER data for broad peaks relevant to therapy are available. Preliminary measurements on radiosensitivity variation as a function of cell cycle indicate that there is a slight reduction in variation at the plateau, compared to X-rays, and a significant reduction at the narrow peak. With the availability of increasing intensity of pion beams, more radiobiological work relevant to radiotherapy is in progress at Los Alamos, Vancouver and Zurich.