MRI features of an intra‐articular histiocytic sarcoma affecting the stifle joint in a dog

A 13‐year‐old, spayed, female German shorthaired pointer was presented for investigation of a right pelvic limb lameness and associated marked stifle swelling. Magnetic resonance imaging revealed a mildly heterogeneous, T2‐weighted hyperintense and T1‐weighted homogeneously hypointense, contrast‐enhancing fine nodular mass arising from the synovium within the joint space of the right stifle, accompanied by adjacent bone marrow oedema characterised by hyperintensity on fat‐suppressed sequences. Based on the imaging characteristics, the primary differential diagnosis was benign villonodular synovitis. Surgical excision with histopathology revealed intra‐articular histiocytic sarcoma. This case highlights that while histiocytic sarcomas involving the stifle joint are documented, their imaging features can closely resemble those of other intra‐articular neoplastic and chronic inflammatory conditions. Consequently, histiocytic sarcoma should remain an important differential diagnosis in dogs with similar clinical and imaging findings.