Morphologic Changes in the Mammary Gland of Megestrol Acetate-treated and Untreated Cats: A Retrospective Study

Abstract

Abnormal mammary gland growth is a side effect of progestin therapy in some cats. In this retrospective study, the nature and significance of morphologic changes in the mammary gland of 17 megestrol acetate (MA)-treated cats were compared to mammary lesions in 97 untreated cats. Fourteen out of 17 MA-treated cats had non-neoplastic mammary lesions including fibroepithelial hyperplasia (nine cats), lobular hyperplasia (three cats), and duct ectasia (two cats); whereas three MA-treated cats had mammary neoplasms including one adenoma and two carcinomas. Although MA has been causally linked to mammary cancer in cats, only mammary fibroepithelial hyperplasia was clearly associated with MA therapy in this study. Fibroepithelial hyperplasia occurred in older (average age 8.1 years) neutered male and female cats in the MA-treated group and in younger (average age 2.1 years) female cats in the untreated group. Morphologically, both intraductal and solid fibroepithelial growth patterns were seen. Intraductal fibroepithelial hyperplasia was further subdivided into papillary and circumferential types. An apparent greater association between MA therapy and the intraductal types of fibroepithelial hyperplasia was noted. Furthermore, it appears likely that mammary lobular hyperplasia also is linked to MA therapy. Possible mammatrophic effects of MA and other growth-promoting agents in the cat are discussed.

Clinical and pathological study of feline mammary fibroadenomatous change associated with depot medroxyprogesterone acetate therapy

“…FMFAC has also been associated with synthetic progestin therapy e.g. medroxyprogesterone acetate (MPA) and megestrol acetate (MA) when used during prolonged periods (months to years) in intact or neutered female and male adult cats (Hayden et al, 1989). MPA is commercially available as a repositol (depot) injectable product used mainly as a contraceptive drug for dogs and cats.…”

Feline Mammary Fibroepithelial Hyperplasia: A Clinical Approach

“…Although necrosis or ulceration are rare [17], they can be found in long lasting situations or whenever the reduction of the mammary gland swelling was attempted by progestin administration. Microscopically, the two patterns are similar [18]: the diseased mammary gland is characterized by the proliferation of glandular fibroepithelial elements. The lesions correspond to well-demarcated, non-encapsulated growths within the mammary gland [17,31], with the ducts forming pseudo-acinar or cystic structures, encircled by a loose, myxoid stroma [21].…”

“…It predominantly affects younger intact female cats, a segment of the population that also presents an increased ratio of spontaneous ovulation [15,16]. The reported age range for FEH is 6 months to 13 years [17][18][19][20]. Not so frequently, the condition may also be seen in aged females, associated or not with a contraceptive treatment, and sporadically in hormonally treated male tomcats.…”

“…Nevertheless, according to our experience, incidence of FEH seems to be in regression among the group of animals submitted to progesterone-based contraception, may be due to the fact that most contraceptive treatments are now based in oral, veterinary drugs (such as Megescat®) instead of human design depot products (like Depo-Provera®). Nevertheless, the medroxyprogesterone acetate and the megestrol acetate are the most frequently reported progestin associated to FEH, in particular when the drug is injected [13,14,18,24].…”

“…Macroscopically, FEH lesions appear as firm, well-circumscribed but unencapsulated masses, that may present two types of macroscopic patterns: the solid type, of smooth-surfaced tissue with scant fluid; and the parenchymal, intraductal pattern, with fluid-filled spaces [18]. The two patterns can be combined in the same lesion, or one of them can predominate over the other.…”