In Vivo Beam Localization by Positron Activation in Pion Therapy

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Abstract
Measurements were made of the absolute yield of the positron emitters 1C and 13N produced by 76-MeV negative pions stopped in thick stacked-foil targets of C (graphite), N (boron nitride), O (water) and a tissue-equivalent mixture. The depth distribution of radioactivity exhibited a distinct peak in the water, boron nitride and tissue-equivalent targets correlating well with the position and width of the pion stopping distribution and with the high-LET component of dose. There were 0.0186 11C and 0.0085 13N nuclei produced per pion stopped in water, and 0.0484 11C nuclei per pion stopped in N. Compared to predictions of an intranuclear cascade calculation the yields of 11C and 13N in the stopping region from secondary particle-induced reactions were low, and do not significantly limit the ability to localize the pion star region. The activities measured for the tissue-equivalent target agreed with the predictions based on the measurements for C, N and O targets and were similar to the yields for water. Utilization of a positron camera to image the radiactivity distribution in the pion stopping region was discussed.