Evidence for an Unconventional Radiosensitivity of Rat 9L Subcutaneous Tumors

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Abstract
The cellular radiosensitivity of rat 9L s.c. tumor cells was determined by measuring cell survival with an in vivo to in vitro colony-forming assay. The radiosensitivity of 9L tumor cells did not vary with changes in tumor weight (0.03-2.0 g), appearance of necrosis or anesthesia use. None of the survival curves for cells from air-breathing rats had the traditional break normally associated with the presence of a hypoxic fraction. The composite survival curve showed a sensitivity (D0 [median lethal dose] = 3.6 Gy [gray]) intermediate between and statistically different from that observed for exponential cells from monolayer cultures (D0 = 1.8 Gy) or single cells from dissociated tumors (D0 = 2.1 Gy), and tumor cells from N2-asphyxiated rats (D0 = 5.3 Gy). There are 4 possible reasons for the intermediate radiosensitivity of in situ 9L s.c. tumors: intercellular contact, diffusion-limited phenomena, perfusion-limited hypoxia and cellular recovery. Perfusion-limited hypoxia or more probably cellular recovery apparently is the major contributor to the observed intermediate cellular radiosensitivity of 9L s.c. tumors.