In vivo beam localization by positron activation in pion therapy

Measurements have been made of the absolute yield of the positron emitters ¹¹C and ¹³N produced by 76-MeV negative pions stopped in thick 'stacked-foil' targets of C (as graphite), N (as boron nitride), O (as water), and a tissue-equivalent mixture. The depth distribution of radioactivity exhibits a distinct peak in the water, boron nitride, and tissue-equivalent targets which correlates well with the position and width of the pion stopping distribution and with the high-LET component of dose. There are 0.0186 ¹¹C nuclei and 0.0085 ¹³N nuclei produced per pion stopped in water, and 0.0484 ¹¹C nuclei produced per pion stopped in nitrogen. These yields are compared to predictions of an intranuclear cascade calculation. The yields of ¹¹C and ¹³N in the stopping region from secondary particle-induced reactions are low, and do not significantly limit the ability to localize the pion star region. The activities measured from the tissue-equivalent target agree with the predictions based on the measurements for carbon, nitrogen, and oxygen targets and are similar to the yields for water. The utilization of a positron camera to image the radioactivity distribution in the pion stopping region is discussed.