Pion cancer therapy: positron activity as an indicator of depth-dose.

Negative pions are useful for radiation therapy of localized carcinoma because of their large end-of-range energy deposition upon capture in nuclei and the potentially accurate spatial localization of the radiation dose. Accurate determination of the depth of penetration of the pion beam in vivo can be accomplished by counting the back-to-back annihilation gamma rays resulting from positron activity induced by the stopped pions.