A case of gastrointestinal stromal tumor with epithelioid-like morphology in a guinea pig (Cavia porcellus).

Although gastrointestinal stromal tumors (GISTs) are rare in guinea pigs, few cases have been reported recently. Here, we present a case of a GIST in a guinea pig that showed atypical cellular morphology with extensive epithelioid-like cells. The tumor was located between the small intestine and the tip of the cecum and originated from the jejunum. Histopathologically, the tumor cells were mixed with epithelioid-like or short spindle-shaped cells with round-to-oval nuclei. Multinucleated cells were also observed. The tumor metastasized to the peritoneal lymph node. Immunohistochemically, the tumor cells were diffusely positive for vimentin and c-kit. A mutation (large deletion) in exon 11 of the KIT gene was detected using PCR-direct sequencing. Including the previous five cases, GISTs in guinea pigs tended to exhibit nuclear atypia rather than the typical morphology observed in canine GISTs, despite being diagnosed as spindle-shaped. Additionally, the present case exhibited more nuclear atypia, which may be a characteristic feature of GISTs in guinea pigs.