Anal sac mast cell tumor in a dog

An 11-year-old castrated mixed-breed dog was diagnosed with a unilateral anal sac mass. Anal sacculectomy was performed without complication. Histopathology of the mass was consistent with a well-differentiated mast cell tumor. Analyses for the Ki-67 protein, KIT expression pattern, and the presence of c-kit mutations were done. Recovery from surgery was unremarkable and repeat staging revealed no evidence of mast cell disease at 4, 8, and 12 mo after surgery. To the authors' knowledge, primary mast cell neoplasia of the anal sac has not previously been reported in the veterinary literature. Key clinical message: Although mast cell tumors of the perineal region are commonly encountered in veterinary practice, the anal sac represents a novel location for this disease. Cytology of anal sac masses can be a useful diagnostic tool to confirm the diagnosis, guide staging, and assist in treatment decisions.