Accelerated reoxygenation of a murine fibrosarcoma after carbon-ion radiation

Purpose: Reoxygenation of a murine tumour after irradiation with carbon ions was investigated and compared to that after gamma-rays. Materials and methods: NFSa fibrosarcoma cells were transplanted into the right hind legs of syngeneic C3H male mice. Conditioning irradiation with either 290MeV/u carbon ions or 137-Cs gamma -rays was delivered to the tumours at 8mm diameter. At given times after irradiation the leg tumours, either clamped or not, received test doses of photons. Diff erences in tumour growth delay between the clamped and non-clamped tumours were interpreted in terms of reoxygenation. A lung-colony assay was used to obtain cell-survival curves. Results: The oxygen enhancement ratio in the NFSa tumour for 74keV mum1 carbon ions was 1.6 while that for gamma-rays was 3.4. The NFSa tumours reoxygenated 4 days after 30Gy of gamma-ray irradiation, but reoxygenated as early as 1 day after 16Gy of carbon ions. Reoxygenation after gamma-rays shortened to 1 day when the tumours were initially clamped for the conditioning irradiation. Conclusions: The fraction of surviving oxic cells in the NFSa tumours is larger after irradiation with carbon ions than with gamma-rays, resulting in accelerated reoxygenation.