Relative Biological Effectiveness of 290 MeV/u Carbon Ions for the Growth Delay of a Radioresistant Murine Fibrosarcoma

The relative biological effectiveness (RBE) for animal tumors treated with fractionated doses of 290 MeV/u carbon ions was studied. The growth delay of NFSa fibrosarcoma in mice was investigated following various daily doses given with carbon ions or those given with cesium γ-rays, and the RBE was determined. Animal tumors were irradiated with carbon ions of various LET (linear energy transfer) in a 6-cm SOBP (spread-out Bragg peak), and the isoeffect doses; i.e. the dose necessary to induce a tumor growth delay of 15 days were studied. The isoeffect dose for carbon ions of 14 and 20 keV/μm increased with an increase in the number of fractions up to 4 fractions. The increase in the isoeffect dose with the fraction number was small for carbon ions of 44 keV/μm, and was not observed for 74 keV/μm. The α and β values of the linear-quadratic model for the radiation dose-cell survival relationship were calculated by the Fe-plot analysis method. The α values increased linearly with an increase in the LET, while the β values were independent of the LET. The α/β ratio was 129±10 Gy for γ-rays, and increased with an increase in the LET, reaching 475±168 Gy for 74 keV/μm carbon ions. The RBE for carbon ions relative to Cs-137 γ-rays increased with the LET. The RBE values for 14 and 20 keV/μm carbon ions were 1.4 and independent of the number of fractions, while those for 44 and 74 keV/μm increased from 1.8 to 2.3 and from 2.4 to 3.0, respectively, when the number of fractions increased from 1 to 4. Increasing the number of fractions further from 4 to 6 was not associated with an increase in the RBE. These results together with our earlier study on the skin reaction support the use of an RBE of 3. 0 in clinical trials of 80 keV/μm carbon beams. The RBE values for low doses of carbon beams were also considered.