Responses of bone marrow and tumor cells to acute and protracted irradiation.

Mice bearing NCTC 2472 tumors (either solid or ascitic) were continuously irradiated at 50 rads/hr in order to simulate a dose rate comparable to that generated by tissue implants. Kinetic and survival data were collected over a 40-hr interval on cells obtained from tumors and also from bone marrow. Bone marrow cellularity decreased rather slowly. At 40 hr it was 10% of the control value. The number of stem cells was reduced much more rapidly; after only 12 hrs it was reduced to 3.8% of the control value. The effectiveness of continuous radiation and of acute irradiation on bone marrow stem cells is similar. Labeling indices decreased with increasing time of irradiation; the mitotic indices dropped initially but tended to recover. Under continuous irradiation the ascites cells accumulated in G2 at a rapid rate. After 40 hr 61% of the cells were in G2 phase (versus 5% in the controls), whereas after an acute irradiation cells temporarily accumulated in GÃŒphase. The time interval during which this accumulation is observed is dose dependent. Survival curves of cells from solid tumors show no evidence of hypoxic cells down to a survival level of 10~3; cells from acutely irradiated tumors yielded biphasic