Increased radiosensitivity of a recurrent murine fibrosarcoma following radiotherapy.

The radiosensitivity of a recurrent tumor following local radiotherapy was investigated in order to determine whether or not the cell(s) surviving radiotherapy are radioresistant. A tumor which recurred in a C3Hf/HeMsNrs male mouse 200 days after local irradiation of a transplanted syngeneic fibrosarcoma was examined. The recurrent tumor showed the same biological properties, such as karyotype, histological features, and ability to produce colony stimulation factor, as the original tumor. On the other hand, the tumor control dose and the D0 value of the recurrent tumor were much smaller than those of the original tumor. Moreover, the recurrent tumor grew more slowly than the original tumor. It was concluded that initial radiotherapy might have changed the intrinsic radiosensitivity of the original tumor cells.