Benign gastric stromal tumour with dual myoid and neural differentiation in the mule

In humans, gastrointestinal stromal tumours (GISTs) represent a distinct category of mesenchy- mal neoplasms sharing a wide spectrum of both mor- phological and immunophenotypical features and a variable clinical behaviour (benign, borderline and malignant). To the best of our knowledge, there are no reports of GISTs in equines. We report the first case of a benign gastric stromal tumour occurring in the stomach of a 12-year-old male mule. The tumour was composed of a prolifera- tion of bland-looking spindle to epithelioid cells arranged in interlacing broad fascicles and set in a loose fibrous to focally myxoid stroma. Nuclear atyp- ia, mitotic activity and necrosis were not seen. Im- munohistochemistry, revealing distinct areas positive for both α-smooth muscle actin and S-100 protein in tumour cells, allowed us to classify the tumour as a “gastric stromal tumour with dual myoid and neural differentiation”. The present case points out the pos- sibility that GISTs may occur in equines with mor- phological features and an immunohistochemical profile similar to those observed in humans.