Out of 24 primary mammary tumors, arising in rats of the WAG/Rij Wistar strain after low dose irradiation, with or without prolonged treatment with estrogen, a slow-growing, well differentiated adenocarcinoma (MCR-83) was selected. This tumor, induced by radiation alone, is independent of estrogen pellets for growth after transplantation into adult female rats, but nontransplantable into males or ovariectomized females. Measurements of tumor growth and contents of both estrogen and progesterone receptors on three successive passages are not indicative of a rapid progression in growth rate or to hormone independency. Ovariectomy and treatment with tamoxifen given a pronounced inhibition of tumor growth, whereas neither methotrexate nor cyclophosphamide is effective. Growth rate is significantly increased when rats are given 17β-estradiol. Flow cytometric DNA analysis as well as in situ S-phase cell detection with anti-bromodeoxyuridine antibodies show a 3-fold increase in S-phase fraction cells within 4 days after the onset of estrogen treatment.

No spontaneous metastases have been found so far, but lung nodules develop after i.v. inoculation of tumor cells. From one of these nodules a fast-growting, hormone independent subline (MCR-86) has been derived, showing both lymphatic and hematogenous dissemination upon s.c. transplantation.

By showing several features of hormone responsive human disease in its early stage of progression the MCR-83 tumor system may be a clinically relevant model for studies on endocrine regulation of tumor growth and its therapeutic manipulation.

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This research was supported by Imperial Chemical Industries grant 50-83-11.

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