Radiobiologic effect of intermittent radiation exposure in murine tumors

In stereotactic irradiation using a linear accelerator, the effect of radiation may be reduced during intermittent exposures owing to recovery from sublethal damage in tumor cells. After our previous in vitro study suggesting this phenomenon, we investigated the issue in murine tumors.

We used EMT6 and SCCVII tumors approximately 1 cm in diameter growing in the hind legs of syngeneic mice. Three schedules of intermittent radiation were investigated. First, 2 fractions of 10 Gy were given at an interval of 15-360 min to investigate the pattern of recovery from sublethal damage. Second, 5 fractions of 4 Gy were given with interfraction intervals of 2.5-15 min each. Third, 10 fractions of 2 Gy were given with interfraction intervals of 1-7 min each. Doses of 15-20 Gy were also given without interruption to estimate the dose-modifying factors. Tumors were excised 20 h later, and tumor cell survival was determined by an in vivo-in vitro assay.

In the 2-fraction experiment, the increase in cell survival with elongation of the interval was much less than that observed in our previous in vitro study. In the 5- and 10-fraction experiments, no significant increase in cell survival was observed after the intermittent exposures. Moreover, cell survival decreased at most points of the 5-fraction experiments by interruption of radiation in both EMT6 and SCCVII tumors. In the 10-fraction experiment, cell survival also decreased when the interruption was 3 or 7 min in EMT6 tumors.

The results of the present in vivo studies were different from those of our in vitro studies in which cell survival increased significantly when a few minutes or longer intervals were posed between fractions. This suggests that recovery from sublethal damage in vivo may be counterbalanced by other phenomena such as reoxygenation that sensitizes tumor cells to subsequent irradiation.