EXPRESS: Lameness caused by intramuscular osteochondroma in semimembranosus muscle in a male cat

Case summary: A 9-year-old male castrated domestic shorthair cat was presented for evaluation of progressive right pelvic limb lameness. Orthopedic examination revealed a weight-bearing lameness of the right pelvic limb, and a mass over the medial aspect of the right stifle, which elicited pain upon palpation. Preoperative complete blood count and serum biochemistry panel were within reference intervals. Radiographs and computed tomography identified a mass at the level of the distal femur, with focal mineralization and enlargement of the right semimembranosus muscle. Incisional biopsy suggested osteochondroma, osteochondromatosis, and chondroma. The mass was marginally excised, with histopathology consistent with an intramuscular osteochondroma. The cat returned to normal activity and is currently healthy and alive 3 years following the surgery without evidence of recurrence. Relevance and novel information: Soft tissue osteochondroma is a rare condition that can occur intramuscularly, and to the authors’ knowledge, this condition has not been previously reported at this location in a feline patient. Based on this report, although considered rare, clinical awareness of this tumor may aid practitioners in identifying intramuscular osteochondroma as a differential for firm intramuscular masses. Intramuscular osteochondroma carries a fair-to-good prognosis dependent on the location in an otherwise clinically healthy cat.