Comparative studies of the metastatic potential of three transplantable rat mammary carcinomas of spontaneous origin.

The metastatic potential of 3 spontaneously arising mammary carcinomas (Sp4, Sp15 and Sp22) has been examined when transplanted in the form of a viable cell suspension into the tissue of origin. Primary tumours were excised at different times after implantation and it was found that the metastatic potential of the immunogenic tumour Sp4 was directly correlated with the size of the primary tumour when excised. By contrast, the incidence of metastases from the non-immunogenic tumours Sp15 and Sp22 was similar irrespective of the size of the primary tumour on excision. The pattern of metastasis also differed between the tumours, although here there was no relation to immunogenicity. Thus, resection en bloc of large primary Sp4 or Sp15 tumours plus regional lymph nodes could be completely curative, signifying initial spread of tumours via the lymphatics and only subsequently via the blood stream. On the other hand, resection en bloc of primary Sp22 tumours plus regional lymph nodes at a similar stage of primary tumour development was never curative, signifying early spread via the blood stream. Other studies showed that the metastatic potential of mammary carcinoma Sp4 was an innate characteristic of the tumour and not related to the tissue of implantation since in addition to metastasizing from the mammary pad it metastasized when implanted either s.c. or intradermally in a region devoid of mammary tissue. Furthermore, a rat sarcoma Mc7 showed a negligible tendency to metastasize when implanted either in the mammary pad or in the s.c. tissue, where it had been induced with methylcholanthrene.