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.
MRI is rarely used to diagnose femorotibial neoplasia in veterinary practice, with CT often preferred for prognostic and surgical planning purposes. In human patients undergoing limb-sparing procedures, MRI remains the imaging modality of choice. In one report on MRI features of proximal tibial osteosarcoma in dogs, MRI was least accurate in predicting tumour length, but effectively identified a clear interface between normal and abnormal tissue within the bone marrow cavity, with T1W images being the most useful for detecting this interface. Nevertheless, MRI provides superior soft tissue contrast, enhancing visualisation of the cruciate ligaments, menisci and surrounding soft tissue structures.
In conclusion, this case is the first published description of the MRI findings of a histologically confirmed histiocytic sarcoma affecting the stifle joint in a dog. It highlights the importance of obtaining a definitive histological diagnosis before initiating treatment, as imaging characteristics may closely resemble those of other intra-articular neoplasms and inflammatory conditions.
