Alteration of Radiosensitivity of Quiescent Cell Populations in Solid Tumors Irradiated with X‐Rays Twice at Various Intervals

5‐Bromo‐2′‐deoxyuridine (BUdR) was injected into SCC VII or EMT6/KU tumor‐bearing mice intraperitoneally to label all the proliferating tumor cells. First, the mice were irradiated with X‐rays at a dose of 10 Gy, followed by a dose of 0–20 Gy at 0, 12, 24 or 48 h later. During the interval, no BUdR was injected. Immediately after the second irradiation, the tumors were excised and trypsinized. The micronucleus (MN) frequency in cells without BUdR labeling was determined by means of incubation with cytochalasin‐B (a cytokinesis‐blocker) and immunoftuorescence staining for BUdR. When the tumors were not pretreated with BUdR before the first irradiation, the MN frequency in all tumor cells was determined. To determine the labeling indices of SCC VII and EMT6/KU tumors at the time of the second irradiation, each group also included mice that were continuously administered BUdR until just before the second irradiation using mini‐osmotic pumps which had been implanted subcutaneously 5 days before the first irradiation. The MN frequency of all tumor cell populations obtained immediately after the second irradiation decreased in proportion to the increase in interval time. However, in both tumor systems, the MN frequency of unlabeled cell populations, which could be regarded as quiescent cells in the tumors at the time of the first irradiation, was raised with increase in the interval time. In addition, the labeling index at the second irradiation was higher than that at the first irradiation. These findings support the occurrence of recruitment from quiescent to proliferating state during fractionated irradiation.