Abstract
Background and purposeThe aim of the study was to evaluate the therapeutic gain of carbon ion (C-ion) radiotherapy using a mouse model. Materials and methodsTransplanted fibrosarcoma (NFSa) growing in C3H/He mice and murine small intestine were irradiated with 290MeV/nucleon C-ion beams (C-ions) in 1–12 fractions separated by 4h. The cell killing efficiencies of C-ions were measured using jejunum crypt survival and tumor growth delay (TGD) assays. ResultsThe equieffect dose for crypt survival and TGD increased with increasing number of fractions after X-rays and 20keV/μm C-ions, whereas TGD after 77keV/μm C-ions rather decreased. Crypts showed stronger LET-dependent increase in α terms than the tumor while β terms less depended on LET irrespective of tissues. Therapeutic gain factor, i.e., a ratio of tumor RBE over crypt RBE, of 77keV/μm C-ions was more than unity at any doses while that of 20keV/μm C-ions increased with an increase in dose per fraction. ConclusionsThese specific data imply that use of large dose per fraction would be suitable for C-ion radiotherapy irrespective of LET from the point of view of therapeutic gain, though small dose per fraction by high-LET radiation decreases total dose for tumor.
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