The effects of local X-irradiation of a solid, rapidly metastasizing sarcoma in the rat on kinetics of dissemination and growth of metastases in lymph nodes and lungs are described. Corresponding dose-effect curves obtained for growth of the primary tumour (Pr) and its metastases in unirradiated tissues showed that local irradiation of Pr caused an exponential decrease in growth of metastases due to any dissemination occurring after irradiation, but was also responsible for stimulating growth of metastases already established before treatment in lymph nodes and in lungs. This stimulating effect was most marked when Pr was larger at the time of treatment and when high doses were given to eradicate Pr. This effect is attributed to the liberation of growth stimulating substances (GSS) from a pool of GSS produced in the irradiated Pr by sterilized, but metabolically active and growing tumour cells (HR cells). This effect of HR cells on tumour growth and metastases was also demonstrated when rats were inoculated with viable tumour cells and subsequently treated by injecting large doses of HR cells prepared in vitro, into tissues remote from the Pr tumour site.
The systemic effects of GSS on metastases were most clearly seen after immunosuppression of recipient hosts by sublethal whole body irradiation, since immunosurveillance in unirradiated rats resulting from a rapidly developing allogenic tumour-host incompatibility caused marked reductions in clonogenicity of the tumour which tended to overshadow the GSS effect. The latter was also masked in immunosuppressed hosts when excessively high rates of dissemination were due to growth of large Pr inocula for sufficiently long to “saturate” the capacity for growth of metastatic tumour in lymph nodes and lungs.
The relevance of these findings to clinical radiotherapy is discussed.