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

Purpose: We analyzed the characteristics of radiosensitivity and poten tially lethal damage repair in the quiescent cell populations of murin e SCC VII solid tumors irradiated with fast neutrons, in comparison wi th those irradiated with 10 MV X rays. Methods and Materials: SCC VII tumor-bearing C3H/He mice were irradiated with 30 MeV fast neutrons or 10 MV X rays after receiving 10 injections of 5-bromo-2'-deoxyuridine (BUdR) to label all proliferating tumor cells. Immediately or 24 h af ter irradiation, the tumors were excised and trypsinized. The tumor ce ll suspensions thus obtained were incubated with cytochalasin-B (a cyt okinesis blocker), and the micronucleus frequency in cells without BUd R labeling was determined using immunofluorescence staining to BUdR. T his micronucleus frequency was then used to calculate the surviving fr action of unlabeled cells from the regression line for the relation be tween micronucleus frequency and the surviving fraction of all tumor t ells. Thus, a cell survival curve could be determined for the cells no t labeled by BUdR, which can be regarded as the quiescent cells for al l practical purposes. Results: The difference in intrinsic radiosensit ivity between all tumor and quiescent cells became smaller by using fa st neutrons, compared with X rays, especially when large radiation dos es were given. Potentially lethal damage repair by quiescent cells was less evident following irradiation with fast neutrons than with X ray s, especially when large doses were delivered. Conclusion: By using fa st neutrons in clinical radiotherapy, the radiosensitivity of solid tu mors is thought to depend on their heterogeneity less critically than for X rays.