A 38-year-old woman with history of rheumatoid arthritis presents with acute right knee pain and swelling. Plain radiographs show joint space narrowing and marginal erosions. MRI of the knee reveals a focal lesion within the medial femoral condyle with low signal intensity on T1-weighted images, high signal intensity on T2-weighted images with peripheral enhancement after gadolinium administration. The lesion measures 1.2 cm in diameter. What is the most likely diagnosis?