Inhibition by retinoid and ovariectomy of additional primary malignancies in rats following surgical removal of the first mammary cancer

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Abstract
Retinoids are highly effective inhibitors of mammary carcinogenesis in the rat. This study was designed to determine the influence of retinyl acetate and bilateral ovariectomy on the rate of new tumor appearance when treatment is started at the time of removal of an animal's first mammary tumor. Fifty‐day‐old virgin female Sprague‐Dawley rats received a single intravenous injection of 35 mg N‐methyl‐N‐nitrosourea per kg body weight. When an animal's first mammary tumor reached a diameter of 0.5 cm it was excised and the animal was entered into a treatment group: (1) intact, placebo diet; (2) intact, 328 mg retinyl acetate/kg diet; (3) ovariectomy, placebo diet; (4) ovariectomy, 328 mg retinyl acetate/kg diet. New tumor incidence was reduced from 94% in Group 1 to 15% in Group 4. Groups 2 and 3 had an intermediate tumor response. These data indicate that retinyl acetate and ovariectomy are active in cancer prevention when treatment is began after removal of a palpable tumor, a time at which preneoplastic lesions are present in the mammary glands.