Co‐existing monophasic teratoma and uterine adenocarcinoma in a female dog

Ovarian teratomas are occasionally reported in dogs; the rarest type is the monophasic teratoma, composed of tissues originating from only one germ layer. Canine endometrial adenocarcinomas are also rare in dogs and mainly affect geriatric females. This report describes the case of co‐existing ovarian teratoma and uterine adenocarcinoma in a 10‐year‐old nulliparous female Boxer presented with lethargy, anorexia and purulent vaginal discharge. Abdominal ultrasonography evidenced pyometra and a mass in the left ovary. This was composed of a uniform whitish tissue with multiple cystic structures. The histology revealed an atrophy of the ovarian parenchyma, compressed by a proliferation of well‐differentiated nervous tissue staining positively to vimentin, S100 and neuronal specific enolase (NSE), and negatively to keratin and inhibin. The left uterine horn, whose diameter was markedly increased, showed foci of endometrial cellular atypia, evident nucleoli and mitoses, at light microscopy. To our best knowledge, this is the first report of a co‐existing ovarian monophasic teratoma and endometrial adenocarcinoma, two rare reproductive neoplasia in dogs.