Radiation response of autochthonous mammary tumors in SHN mice.

The radiation response of spontaneously induced autochthonous mammary tumors was studies in SHN mice, a strain developed at the National Cancer Center. Tumors were mostly adenocarcinomas and grew with an average volume- doubling time of 3.0 days. When tumors reached a size of 8 approximately 10 mm in diameter, they were given a single, local irradiation with 6 MVp X-rays generated by a medical linear accelerator. With radiation doses lower than 2 krad, the tumours regrew after a temporary interruption. Surviving fractions of tumor cells in situ were estimated from the tumor cells in situ were estimated from the tumor regrowth times, yielding a survival curve with D0=480 rad for n=2. With 6.5 krad, tumor regression was very rapid: the volume-halving time was 1.7 days, and temporary tumor control was achieved. This rapid radiation response was not necessarily correlated with the radiosensitivity of tumor cells and is thought to be related to the structure of the mammary tumor mass. A disadvantage of this mouse strain was its high multiple tumor incidence which interfered with the observation of tumor control for adequate periods after irradiation.