Feline cutaneous mast cell tumor with extensive macrophage-associated cell-in-cell structures.

An 8-y-old, castrated male Korean Shorthair cat was presented to a veterinary clinic for evaluation of a cutaneous mass on the left inner thigh. Microscopic examination revealed a poorly circumscribed dermal neoplasm composed of poorly granulated, pleomorphic neoplastic mast cells with frequent cell-in-cell structures. Notably, viable neoplastic mast cells were engulfed by large macrophages, indicating a specific form of the cell-in-cell phenomenon. Immunohistochemistry for c-KIT, IBA1, and cleaved caspase-3 was used to support the diagnosis. To our knowledge, cell-in-cell phenomenon has not been previously described in feline cutaneous mast cell tumors, nor has the clinical outcome of such cases been reported.