A subcutaneous mass on the neck of a horse

A 25-year-old Arabian gelding was presented for investigation of a subcutaneous neck mass. Fine-needle aspirates and impression smears revealed mast cells with widely varying degrees of cytoplasmic granulation and scattered eosinophils. Histopathology revealed a poorly circumscribed mass composed of sheets and bundles of mast cells with a large population of eosinophils. The mast cells were separated into numerous lobules by a heavy collagenous stroma, and multifocal collagen necrosis was present. Strong reactivity of the tumor cells for both Giemsa and toluidine blue stains confirmed the diagnosis of a mast cell tumor, and a Luna stain accentuated the eosinophilic infiltrates. Cutaneous mast cell tumors are found in many domestic animals but are uncommonly encountered in horses. Equine cutaneous mast cell tumors are usually benign, and there are no reports of visceral metastasis. Surgical excision is thought to be curative.