Mouse skin reactions following fractionated irradiation with carbon ions

Purpose: Skin reactions in the mouse leg following various daily doses given with 290 MeV/u carbon ions were investigated. Materials and methods: Seven different LET (linear energy transfer) values ranging from 14 to 100 keV/mu m were selected. The fractionation schedules were 1-, 2-, 4- and 8-daily fractions. The isoeffect doses to produce moist desquamation on the dose-response curves were calculated with 95% confidence limits. Results: The isoeffect doses for carbon ions of 14 and 20 keV/mu m increased with an increase in the number of fractions up to 4 fractions, but became constant when the number of fractions further increased to 8 fractions. This leveling off in isoeffect dose was more prominent for 40 keV/mu m. Recovered dose per fraction was largest for 2 fractions of the 14 keV/mu m carbon beam. The isoeffect doses for 50, 60, 80 and 100 keV/mu m consistently increased with an increase in the number of fractions and did not show saturation up to 8 fractions. RBE (relative biological effectiveness) increased linearly with LET for all fractionation schedules. Conclusions: These results suggest that daily fractionation with carbon ions could spare radiation damage in patients, and that changes the fractionation schedule affect clinical outcome.