Metastasis and the excision of irradiated LMCI tumours in the rat.

Lymph node metastases occurring within 150 days of local non-curative irradiation and excision of a transplantable mammary adenocarcinoma (LMCI) have been scored and their growth measured in isogeneic rats. Single doses (5-40 Gy) of 60Co gamma-rays were given to the primary at 8-10 mm diameter and these were excised either immediately or up to 30 days later. From the results it is concluded that approximately half of all rats have occult dissemination at the time of irradiation. Only these form metastases after the early excision of 10-40 Gy treated tumours but the number of positive sites per animal is significantly reduced. Delayed excision of the irradiated primary permits the further seeding of metastases and, with its regrowth, the incidence of positive rats and the mean number of metastases per animal is restored to that observed after control surgery. This process was dependent on the radiation-induced delay in growth of the primary tumour. However, all metastases, irrespective of site, radiation dose, and time of primary tumour excision showed a growth rate characteristic of the untreated LMCI tumour. No evidence was obtained for an enhancement in the dissemination of metastatic cells from the irradiated but regrowing primary tumour. However, an alteration in the pathways of lymphatic dissemination leading to an increased number of metastases seeding the mediastinum and abdominal lymph nodes may have occurred.