Negative pions in radiotherapy: a brief review

The frequency and energy distribution of fast-neutron interaction products such as protons, alpha particles, and heavy recoils and those arising from negative pion absorption processes are similar. Because of these similarities in radiation quality of negative pions and fast neutrons in the treatment volume, the therapeutic experience of fast neutrons is relevant for negative pions as well. However, in the case of negative pions, dose deposited in the treatment volume extends to much lower LET values than for fast neutrons because a significant fraction of the dose is deposited by energy losses of negative pions. The biological effectiveness for nearly monoenergetic negative pions at the peak of the depth-dose distribution was found to be significantly higher than at the entrance. For pion beams of larger momentum spread that are therapeutically relevant, the differences in biological effectiveness are reduced considerably; however, a significant reduction in OER still can be expected. Considerable changes in biological effectiveness within the peak region have been found. These changes will be reduced when opposed fields of negative pions are used. Although the current knowledge on biological effects sheds some light regarding what to expect when negative pion beams are used in therapy, more work needs to be done with intense negative pion beams before its possible therapeutic use.