Primary intraosseous mast cell tumour of the third phalanx in a quarter horse.

Few cases of equine mast cell tumour have been reported in the literature. Most describe solitary nodular lesions of the skin and subcutaneous tissue that occasionally involve the underlying musculature (Altera and Clark 1970; Doran and Collins 1986; Nyrop et al. 1986; McEntee 1991). Lesions are generally confined to soft tissues and appear to be hyperplastic rather than neoplastic. One case involving local invasion of bone by a cutaneous neoplasm has been reported (Riley et al. 1991). The present report describes a case of hindlimb lameness due to a pathological fracture of the third phalanx resulting from a primary intraosseous mast cell tumour. This unique clinical presentation demonstrates that equine mast cell tumours are not always benign soft tissue lesions.