Qualitative Alteration in Radiation Injury Under Hypoxic Conditions

The nature of the absorption of x and gamma rays is such that a significant portion of the energy is dissipated in dense ion clusters with a high linear energy transfer (LET) (1). Such high LET radiations have been called the α component by Widerö (2). Oxygen is known to enhance the effects of low LET radiations (3, 4); hence, in the absence of oxygen the relative importance of the high LET or α component of x and gamma rays would increase. Since high LET radiations produce injury which apparently differs from that of low LET radiations in the degree of repair demonstrated by split-dose studies (5), it might be expected that repair of injury caused by x or gamma rays under hypoxic conditions would be decreased. Any decrease in repair of injury would be important in radiotherapy, since it would increase the killing of hypoxic tumor cells relative to the euoxic when fractionated treatment is used. Previous investigations in our laboratory with the mouse LD50/30 dose (6) and the endogenous spleen colony technic (7) demonstrated a decrease in early recovery between doses when hypoxic cells were irradiated with 250 kVp x rays. The earlier studies did not establish complete dose response curves and did not prove that severe hypoxia was obtained in vivo by exposure of mice to 5 per cent oxygen. Since the LET spectra of 60Co gamma rays and 200 kV x rays differ (8), it also seemed important to investigate the effect of radiation quality on repair of sublethal injury. The present experiments demonstrate that a qualitative change in injury under hypoxic conditions does occur with 60Co radiations and that the in vivo method does produce severe hypoxia with true alterations in the results of fractionated radiation doses. Material and Methods Female LAF1 mice, aged twelve to sixteen weeks, from the Jackson Laboratories, Bar Harbor, Maine, were used in all experiments. They were housed 9 per cage and fed ordinary laboratory chow. The spleen colony technic of Till and McCulloch (9) was employed to measure cell survival and recovery with split doses. In the endogenous technic the mouse is exposed to radiation doses in the sublethal ranges that allow survival of hematopoietic colony-forming units (CFU) which can be recognized in the spleen. Radiation dose-response curves were determined from spleen colony counts made ten days after the exposure (Diagram A). The exogenous CFU technic involved injection of bone marrow from donor mice to irradiated recipients by the intravenous tail vein route. The recipients received a sufficient total-body dose (1,000 rads) to prevent endogenous colony formation and to permit the growth of transplanted colony-forming cells in the spleen.