Canine smooth muscle tumours of soft tissue: a series of 23 cases.

Introduction: Canine smooth muscle tumours (SMTs) occur mostly in the gastrointestinal and female genital tracts, while soft tissue SMTs are reported rarely. In man, soft tissue leiomyomas are prevalent in cutaneous and subcutaneous tissues, while leiomyosarcomas develop more frequently in retroperitoneal, pelvic or intra-abdominal tissues in women. This study describes the clinicopathological features of canine soft tissue SMTs. Materials andMethods: Cases were collected retrospectively, clinical data retrieved and diagnoses confirmed by re-evaluation of HE-stained slides and by immunohistochemistry. Mitotic count, atypia, necrosis and histological grade were assessed.Oestrogen (OR) and progesterone (PR) receptor expression was evaluated immunohistochemically. Results: Twenty-three cases were collected: eight leiomyomas and 15 leiomyosarcomas (seven grade I, six grade II, two grade III). Thirteen dogs were female (5 neutered) and 10 male (one neutered). Eleven SMTs were subcutaneous (five in the limbs, four perineal, two perianal); five were pelvic, five retroperitoneal, one intra-abdominal and one in the oral cavity. Seven of 10 retroperitoneal/pelvic SMTs were found in female (70%) and three in male dogs. Two perineal leiomyosarcomas and one pelvic leiomyoma expressed OR. One retroperitoneal leiomyoma expressed PR. All OR- and/or PR-positive SMTs occurred in intact females. Conclusions: Canine soft tissue SMTs were more frequently subcutaneous (limbs and perianal/perineal region) and retroperitoneal (including pelvic). Retroperitoneal SMTs were more common in female dogs, paralleling human SMTs. OR and PR expression in some cases suggests the existence of a distinct subset of canine soft tissue SMTs that may display different behaviour, to be assessed in future studies.