Hindawi Publishing Corporation Case Reports in Medicine Volume 2011, Article ID 491470, 6 pages doi:10.1155/2011/491470 Case Report Giant Cell Tumor of the Pes Anserine Bursa (Extra-Articular Pigmented Villonodular Bursitis): A Case Report and Review of the Literature Haitao Zhao,1 Aditya V. Maheshwari,2, 3 Dhruv Kumar,4 and Martin M. Malawer2, 5 1 Department of Orthopedic Oncology, Beijing JiShui Tan Hospital, Peking University, 31 Xinjiekou East Street, Xicheng District, Beijing 100035, China 2 Department of Orthopedic Oncology, Washington Hospital Center, 110 Irving Street North West, Washington, DC 20010, USA 3 Department of Orthopedic Surgery, SUNY Downstate Medical Center, 450 Clarkson Avenue, P.O. Box 30, Brooklyn, NY 11203, USA 4 Department of Pathology, Washington Hospital Center, 110 Irving Street North West, Washington, DC 20010, USA 5 Department of Orthopedic Oncology, The George Washington University Hospital, 900 23rd Street North West, Washington, DC 20037, USA Correspondence should be addressed to Haitao Zhao, zhuomaxixi@163.com Received 14 February 2011; Accepted 7 April 2011 Academic Editor: Edward V. Craig Copyright © 2011 Haitao Zhao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Pigmented villonodular synovitis (PVNS) is a rare, benign, proliferating disease aﬀecting the synovium of joints, bursae, and tendon sheaths. Involvement of bursa (PVNB, pigmented villonodular bursitis) is the least common, and only few cases of exclusively extra-articular PVNB of the pes anserinus bursa have been reported so far. We report a case of extra-articular pes anserine PVNB along with a review of the literature. The lesion presented as a painful soft tissue mass in the medial part of the proximal leg. A magnetic resonance imaging showed areas of low to intermediate signals in all sequences and the lesion enhanced heterogeneously with contrast. Diagnosis was conﬁrmed by an incisional biopsy, and an intralesional resection was performed. The postoperative course was uneventful, and the patient is free of disease with no functional deﬁcit at 2 years followup. As with other rare lesions, clinical and radiographic ﬁndings in addition to histological examination are essential for correct diagnosis. 1. Introduction Pigmented villonodular synovitis (PVNS) is a rare, benign, proliferating disease aﬀecting the synovium of joints, bursae, and tendon sheaths. Jaﬀe et al. [1] regarded the synovium of the joint, tendon sheath, and the bursa as an anatomic unit that can give rise a common family of lesions, includ- ing localized and diﬀuse forms of pigmented villonodular synovitis (PVNS), giant cell tumor of the tendon sheath (nodular tenosynovitis), and the extra-articular pigmented villonodular synovitis arising from the bursa (pigmented villonodular bursitis (PVNB) or diﬀuse giant cell tumor of the tendon sheath). Involvement of bursa is the least common, and only few cases of exclusively extra-articular PVNB of the pes anserinus have been reported so far [1– 11]. The purpose of this paper is to report an additional case of extra-articular pes anserine PVNB along with a review of literature. The patient was informed that data from the case would be submitted for publication, and she consented. 2. Case Report A 28-year-old female presented with a slowly enlarging intermittently painful mass over the medial aspect of her proximal right leg for the past three months. She denied any antecedent trauma. She denied any locking, clicking, loss of motion, or increased warmth around her knee. She denied any constitutional symptoms or any functional disability. On examination, there was a 4.0 × 3.0 cm relatively ﬁrm, tender mass on the medial aspect of the proximal leg at the level of the insertion of pes anserinus. The mass appeared ﬁxed to the underlying tibia with no overlying warmth, erythema,