Evidence for an Unconventional Radiosensitivity of Rat 9L Subcutaneous Tumors

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.