Mammary Nodules in Dogs During Four Years' Treatment With Megestrol Acetate or Chlormadinone Acetate

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Abstract
Mammary nodules were evaluated during the first 4 years of a chronic toxicity study involving female beagle dogs treated for 7 years with megestrol acetate or chlormadinone acetate. The evaluation included clinical observations and histopathologic examination of tissues from dogs killed at 2 and 4 years. Dosages were 1, 10, and 25 times the projected human dose of megestrol and 25 times the projected human dose of chlormadinone. Clinically, numerous palpable mammary nodules were found in dogs treated with middle and high doses of megestrol and with chlormadinone. Of 38 grossly detected nodules evaluated microscopically from the megestrol-treated dogs, 27 were nodular hyperplasias, 5 were benign mixed mammary tumors, 3 were ductal dilatations, 1 was a lymph node, 1 was fat necrosis, and 1 was apparently the umbilicus. Twenty-two nodules from chlormadinone-treated dogs included 12 nodular hyperplasias, 4 benign mixed mammary tumors, 3 lymph nodes, 1 chondromucoid degeneration, and 1 adenocarcinoma; in 1 specimen, no nodule or mammary tissue was found. Involution, regression, and sclerosls of many areas of nodular hyperplasia were evident at 4 years. These data suggest that the number of clinicallf palpable mammary nodules can be misleadmg and is not necessarily an index of neoplasia. Benign mixed mammary tumors and the 1 adenocarcinoma from the chlormadinone-treated dog appeared as distinct entities and were not related developmentally to nodular hyperplasia.

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