RADIOISOTOPES

Heavy ion radiotherapy is expected to reduce the risk of second cancer by decreasing exposure of normal tissues to radiation. Emerging studies have attempted to predict the risk of second cancer after carbon ion radiotherapy based on dose assessment, the epidemiologically-identified risk of photon radiation, and assumptions about the relative biological effectiveness (RBE) of carbon ions. Nevertheless, large uncertainty remains in the choice of RBE of carbon ions in inducing cancer. The present article summarizes carcinogenesis experiments performed in animals using HIMAC. These animal experiments have yielded RBE values for selected tissues, beam types, and age at the time of irradiation. The results indicate potentially variable RBE which depends on tissues, ages, and dose levels. A few additional studies have attempted to identify molecular alterations in tumors induced by carbon ions. Thus, more comprehensive animal carcinogenesis studies are needed.