Long-term survival of a cat with a metastatic gastrointestinal stromal tumor

Case summary: An 8-year-old spayed female mixed-breed cat presented with a 2-year history of recurrent vomiting and hyporexia. Physical examination revealed a palpable mass in the mesogastric abdominal region without discomfort upon touch. Abdominal ultrasonography revealed an intramural mass in the small intestine (duodenum) that caused a decrease in the segment lumen. A complete blood count revealed leukocytosis with marked neutrophilia, eosinophilia and mild hypoalbuminemia. Enterectomy was performed with 3 cm margins and end-to-end anastomosis at the duodenal site of the mass. Histology revealed neoplastic elongated spindle cells that were poorly confined, originating within the muscle layers and that had a mitotic index of 2/10 high-power field (hpf) (at 400×), human epidermal growth factor receptor-type 2 (HER-2) supporting the diagnosis of low-grade sarcoma. Immunohistochemical analysis was positive for KIT, confirming a gastrointestinal stromal tumor (GIST) with a Ki67 level of 15%. Furthermore, multikinase profile biomarkers revealed that the neoplastic cells expressed HER-2 (65%), epidermal growth factor receptor-1 (50%), vascular endothelial growth factor receptor 2 (35%), platelet-derived growth factor receptor beta (15%) and c-KIT (15%). Six months after the original surgery, CT revealed presumptive hepatic, splenic and peritoneal metastases. Toceranib phosphate was prescribed at a dose of 2.75 mg/kg and progressive disease was observed at 8 weeks of follow-up. Relevance and novel information: To the best of our knowledge, this is the first case report to characterize the proliferation biomarker profile of a feline GIST in veterinary oncology. However, despite KIT expression in this tumor, the target drug did not inhibit tumor proliferation, providing new insights into this rare tumor in the feline species.