ALTERATION OF RADIOSENSITIVITY OF QUIESCENT CELL-POPULATIONS IN SOLIDTUMORS IRRADIATED WITH X-RAYS TWICE AT VARIOUS INTERVALS

5-Bromo-2'-deoxyuridine (BUdR) was injected into SCC VII or EMT6/KU tu mor-bearing mice intraperitoneally to label all the proliferating tumo r cells. First, the mice were irradiated with X-rays at a dose of 10 G y, followed by a dose of 0-20 Gy at 0, 12, 24 or 48 h later. During th e interval, no BUdR was injected. Immediately after the second irradia tion, the tumors were excised and trypsinized. The micronucleus (MN) f requency in cells without BUdR labeling was determined by means of inc ubation with cytochalasin-B (a cytokinesis-blocker) and immunofluoresc ence 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 t umors at the time of the second irradiation, each group also included mice that were continuously administered BUdR until just before the se cond irradiation using mini-osmotic pumps which had been implanted sub cutaneously 5 days before the first irradiation. The MN frequency of a ll tumor cell populations obtained immediately after the second irradi ation decreased in proportion to the increase in interval time. Howeve r, in both tumor systems, the MN frequency of unlabeled cell populatio ns, which could be regarded as quiescent cells in the tumors at the ti me of the first irradiation, was raised with increase in the interval time. In addition, the labeling index at the second irradiation was hi gher than that at the first irradiation. These findings support the oc currence of recruitment from quiescent to proliferating state during f ractionated irradiation.