Spontaneous uterine leiomyosarcoma in a rhesus macaque.

A 12-year-old female Rhesus macaque (Macaca mulatta) presented with a uterine enlargement during physical exam. Exploratory celiotomy revealed an intrauterine mass that extended into the cervix. A cystic lesion surrounded and enveloped the left ovary. Because of numerous adhesions and the size of the mass, the animal was euthanized. Microscopically, the neoplasia showed small to medium-sized spindle-shaped cells which formed bundles. The cells resembled smooth muscle, with elongated nuclei, indistinct nucleoli, and scant to moderate quantities of eosinophilic cytoplasm. Mitotic activity was low (0 to 2 mitotic figures/high-power field). Immunohistochemistry revealed that the cells stained for smooth muscle actin. In light of the biological activity (locally invasive) and immunohistochemical findings, a diagnosis of leiomyosarcoma of the myometrium was made. Grossly and histologically, uterine leiomyosarcoma can resemble uterine leiomyoma. Although uterine leiomyosarcoma represents a small percentage of uterine tumors, it should always be considered as a rule out for uterine leiomyoma (fibroids).