THE RESPONSE OF QUIESCENT CELL-POPULATIONS IN MURINE SOLID TUMORS TO IRRADIATION WITH FAST-NEUTRONS

5-bromo-2'-deoxyuridine (BUdR) was injected into SCC VII tumor-bearing mice intraperitoneally to label all proliferating tumor cells. The mi ce mere irradiated with fast neutrons or x-rays. Immediately, or 24 h after irradiation, the tumors were excised, minced and trypsinized. Th e tumor cell suspensions were incubated with cytochalasin-B (a cytokin esis blocker). The micronucleus frequency was determined using immunof luorescence staining to BUdR. The cells that were not labeled with BUd R could be regarded as the quiescent cells. The micronucleus frequency in total tumor cells was determined from the irradiated tumors that w ere not pretreated with BUdR. The difference in radiosensitivity betwe en total and quiescent cells was markedly reduced with fast neutrons, especially at higher doses of radiation. Potentially lethal damage rep air by total and quiescent cells was inhibited more strongly with neut rons than with x-rays. When using fast neutrons, the radiosensitivity of solid tumors depends on their heterogeneity less critically than fo r x-rays.