The radiosensitivity of quiescent cell populations in murine solid tumors in irradiation with fast neutrons

We analyzed the characteristics of radiosensitivity and potentially lethal damage repair in the quiescent cell populations of murine SCC VII solid tumors irradiated with fast neutrons, in comparison with those irradiated with 10 MV X rays.

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 after irradiation, the tumors were excised and trypsinized. The tumor cell suspensions thus obtained were incubated with cytochalasin-B (a cytokinesis blocker), and the micronucleus frequency in cells without BUdR labeling was determined using immunofluorescence staining to BUdR. This micronucleus frequency was then used to calculate the surviving fraction of unlabeled cells from the regression line for the relation between micronucleus frequency and the surviving fraction of all tumor cells. Thus, a cell survival curve could be determined for the cells not labeled by BUdR, which can be regarded as the quiescent cells for all practical purposes.

The difference in intrinsic radiosensitivity between all tumor and quiescent cells became smaller by using fast neutrons, compared with X rays, especially when large radiation doses were given. Potentially lethal damage repair by quiescent cells was less evident following irradiation with fast neutrons than with X rays, especially when large doses were delivered.

By using fast neutrons in clinical radiotherapy, the radiosensitivity of solid tumors is thought to depend on their heterogeneity less critically than for X rays.