Histologic features of mammary carcinomas in zoo felids treated with melengestrol acetate (MGA) contraceptives.

Melengestrol acetate (MGA), a potent synthetic progestin, has been used as a contraceptive in zoo felids since 1975. Mammary gland carcinomas have been linked to MGA treatment in zoo felids, but the histologic features of these tumors and steroid receptor expression have not been described. Zoo felid mammary tumors were requested from participating zoos from 1986 through 1998, and 31 mammary carcinomas from 28 MGA-treated and 3 untreated felids were received. The carcinomas were evaluated on the basis of histologic pattern, tumor grade, and occurrence of metastasis; then features of the tumors were compared to determine if carcinomas in MGA-treated felids differed from those that occur spontaneously. Estrogen- and progesterone-receptor expression was evaluated in 17 of the 31 carcinomas. Of the 31 tumors, 22 (70.9%) had multiple histologic patterns, 29 (93.5%) were high grade, and 28 (90.3%) had metastasized. Within tumors, the tubulopapillary pattern was most common (87.1%, n = 27); solid (61.3%, n = 19), cribriform (38.7%, n = 12), and comedone (25.8%, n = 8) patterns were less common; and the mucinous (3.2%, n = 1) pattern was rare. Both MGA-treated and untreated zoo felids had similar patterns and grades of mammary gland cancer as well as prevalence of metastasis. These results indicate that mammary carcinomas in zoo felids are high grade with a predominant tubulopapillary pattern and aggressive behavior. Five of 17 carcinomas expressed progesterone receptors, and 1 of 17 expressed estrogen receptors. Although more zoo felids with cancer had been exposed to MGA in this study, mammary carcinomas were similar in appearance and behavior in untreated and MGA-treated zoo felids. The association of MGA with the development of malignant mammary gland tumors should be considered when using this contraceptive in zoo felids.