A Large Calf Mass in a Patient with Rheumatoid Arthritis: Giant Pannus Filling Baker’s Cyst

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Abstract Rheumatoid arthritis (RA) presents a significant challenge among autoimmune diseases, affecting millions globally with diverse clinical manifestations, primarily targeting joints and inducing inflammation, pain, and deformities. In some cases, it leads to the formation of Bakers cysts, fluid-filled bursae in the popliteal region, resulting from chronic inflammation of the synovial membrane and subsequent pannus growth. Our case, a young female patient with RA, presenting with calf mass, highlights this complex interplay. She presented with large swelling and pain in her left calf region which on high-frequency ultrasound revealed a large, well-defined heteroechoic mass lesion distending bursa between the medial head of the gastrocnemius and semimembranosus muscle, measuring 25 cm craniocaudally, without any color Doppler abnormality. Ultrasound-guided biopsy was performed to rule out the possibility of sarcoma and it revealed inflammatory changes. Final diagnosis of excessive pannus formation within the Baker’s cyst forming large mass was made, with RA identified as the underlying cause. The differential diagnosis of masses in the popliteal fossa includes synovial/meniscal/ganglion cysts, lipomas, vascular pathologies, and neoplastic conditions. High-resolution ultrasound is an important diagnostic modality for evaluating popliteal fossa masses. Magnetic resonance imaging is typically performed to assess for neoplastic causes and internal derangement of the knee. Our case of a massive pannus filling a Baker’s cyst in the context of RA emphasizes the intricate nature of the disease and its diverse clinical expressions. It is important for the radiologist to be aware of this imaging appearance for timely diagnosis and management.