The tumor bed effect: dependence of tumor take, growth rate, and metastasis on the time interval between irradiation and tumor cell transplantation.

Experiments were designed to investigate the influence of time interval between leg irradiation and tumor cell transplantation on 3 different aspects of the tumor bed effect (TBE): tumor take, growth rate, and metastasis formation. MCA-4 tumor cells were injected subcutaneously into the legs of syngeneic C3Hf/Kam mice that had been locally irradiated with 30 Gy gamma rays 1, 50, 100, or 200 days previously. Interim TD50 values were higher in the day 1, 50, and 100 preirradiated mice than in controls for about 100 days after tumor cell transplantation. However, the final TD50 values determined 220 days after cell transplantation were marginally lower than controls in the day 1, and 50 preirradiated groups. TD50 values in the day 200 preirradiated group were similar to those of controls at all times after tumor cell injection. Retardation of tumor growth rate was observed in all preirradiated groups, but with a progressive decrease in effect as the time between irradiation and tumor cell injection was increased. In the day 200 preirradiated group it was noted that the degree of tumor growth rate retardation decreased as the number of injected tumor cells was increased. The incidence of lung metastases when the "primary" tumor reached a size of 20 mm was higher than in controls in mice irradiated 1, 50, and 100 days before tumor cell injection, but not in the 200-day preirradiated group. Thus, in this tumor system TBE influences tumor take, growth rate, and lung metastasis formation differently; but for all parameters, the effect decayed with increasing time between irradiation and tumor cell transplantation.