Treatment of an invasive equine mast cell tumour in the extensor carpi radialis by extensive tenomyectomy and local corticosteroid injections

Summary Equine mast cell tumours ( MCT ) have been reported to occur in the skin, respiratory tract, oral cavity, synovial structures and eye. These tumours are typically locally invasive and nonaggressive. In this case, a MCT was located in the extensor carpi radialis ( ECR ) and was highly infiltrative. Excision of the ECR and local corticosteroid injection was curative and resulted in a good functional outcome. A 12‐year‐old Thoroughbred gelding presented for lameness, swelling and pruritus associated with the right antebrachium. Sonographic changes were suggestive of severe tenosynovitis and purulent material in the ECR . Surgical exploration revealed maceration at the ECR musculotendinous junction. The extensor sheath contained a large quantity of caseous material and serosanguinous fluid. The ECR tendon and 75% of the muscle was excised. Cytology and histopathology were consistent with a MCT that infiltrated the surgical margins. Post‐operatively the horse was treated with multiple local corticosteroid injections along the incision. Right forelimb movement was initially limited, however, there was steady improvement over 3 months with increasing exercise. At 18 months post‐operatively, no lameness was detectable in the walk, trot or canter and no recurrence of MCT was apparent. The horse appeared to compensate for loss of the ECR by using the biceps during the swing phase. Partial excision and local corticosteroid treatment can be curative for cases of invasive MCT . Extensive excision of the ECR is a viable treatment option for neoplasia or other conditions affecting the ECR in the horse.