| Abstract;The NFSa fibrosarcomas transplanted and growing in the right hind legs of syngeneic C3H male mice were irradiated with 74 keV/.MU.m carbon ions. Tumors were removed, trypsinized and served for lung colony assay. Surviving fraction immediately after a carbon dose of 10 Gy was 0.0058, and similar to that after a gamma dose of 25 Gy, i.e., 0.0056. Surviving fraction increased to 0.037 and 0.021 within 24 hr after carbon and gamma irradiation, respectively. The NFSa tumor cells could, therefore, repair potentially lethal damage caused by carbon ions as efficiently as by .GAMMA. rays. Survivng fractions for both carbon ions and .GAMMA. rays unchanged between Day 1 and Day 3, but rapidly increased from day 4 reaching to 0.5 by Day 7. It is concluded that there was no difference in the recovery kinetics of survived clonogenic cells between carbon ions and .GAMMA. rays. We have raised a question whether fractionated irradiation with low LET carbon ions results equal effects for each fraction. A dose of 5.19 Gy of 20 keV/.MU.m carbon ions was daily delivered to the right hind legs of C3H female mice. The isoeffect total dose was 32 Gy after single dose, and did not change when the number of fractions increased to 2. However, the isoeffect total dose sharply increased to 40 Gy after 3 fractions and then again unchanged till 5 fractions. The sharp increase was not detected for .GAMMA.-ray irradiation. The isoeffect top-up dose decreased with 5.7 Gy/fraction. It is concluded that skin cells recover from carbon-ion damage by a mechanism different from .GAMMA.-ray damage, and implied that change of fractionation schedule in clinical trials results prominent difference in normal tissue damage. (author abst.) |
