Sarcoma resembling a feline injection site sarcoma along the scar of previous forequarter amputation for the treatment of distal humeral osteosarcoma in a cat

Abstract A 9‐year‐old, male, neutered, domestic shorthair cat underwent a right forequarter amputation for osteoblastic osteosarcoma of the distal humerus. Seven months post‐operatively, the cat was reassessed for a firm, semi‐fixed mass along the surgical scar. Computed tomography revealed a right‐sided mass involving the latissimus dorsi and pectoralis muscles. Incisional biopsy revealed a monomorphic population of mesenchymal cells consistent with a sarcoma. A wide surgical excision of the mass was performed. Histopathology revealed a high‐grade sarcoma that could not be differentiated from a feline injection site sarcoma. No evidence of osteoid formation was noted. Five months post‐operatively, a firm, 1.5 cm mass along the incision site and a new 2.8 × 7.1 cm mass in the right medial inguinal region were noted. Cytology of the inguinal mass identified mesenchymal cells. Due to suspected recurrence and development of a second mass, palliative care was elected.