ABSTRACT
Radiation-induced growth delay was measured in rat rhabdomyosarcoma tumors exposed to split doses of high-LET (linear energy transfer) neon ions in the extended-peak ionization region and low-LET X rays. Top-off doses of 7.5, 15, and 25 Gy of 225-kVp X rays were administered to the tumors at 0.5, 4.0, and 24.0 hr following priming doses of either peak neon ions or X rays. The priming doses used were 7 Gy of peak neon ions and 20 Gy of X rays, both of which produced a 10 day delay in tumor regrowth to a volume twice that measured on the day of irradiation. The tumor response to split doses of X rays indicated rapid repair of sublethal damage, with significant recovery occurring at 0.5 hr and complete recovery by 4 hr after the initial 20-Gy X ray dose. The top-off doses of X rays required to produce an additional 10 or 20 days of tumor growth delay were 18 and 7% larger, respectively, when the priming dose was 20 Gy of X rays as compared to 7 Gy of peak neon ions. This result indicates that relatively little interaction of the neon-ion and X ray radiations occurred, even when the time interval between split-dose irradiations was as short as 0.5 hr. Our data indicate that the interaction of high- and low-LET radiation modalities is small, and approaches a simple additivity of effects when the tumors repair a major portion of the sublethal radiation injury imparted by a priming dose before the second dose is administered.
