Invasive Tendon Sheath Fibrosarcoma Causing Radial Osteolysis in a Golden Retriever

This case report details a previously undescribed malignancy of the tendon sheath in a golden retriever. This dog originally presented with lameness of the left forelimb, at which point radiographs revealed a monostotic, lytic lesion of the distal radius with overlying soft-tissue swelling. A fine-needle aspirate was performed, and cytology was compatible with a sarcoma, with the primary differential being an osteosarcoma. After amputation, the leg was submitted for histopathology, which revealed inconsistencies with atypical osteosarcoma lesion, including lack of osteoid deposition. Second opinion histopathology showed a fibrosarcoma that appeared to have originated in the tendon sheath of an extensor tendon and then secondarily invaded the radius. At the time of publication, -17 mo after amputation, the dog continues to do well without any evidence of recurrent or metastatic disease. (J Am Anim Hosp Assoc 2021; 57:285-289. DOI 10.5326/JAAHA-MS-7085)