2 Case Reports in Medicine (a) (b) (c) Figure 1: MRI with (a) axial T1 (TR 602, TE 40), (b) axial short tau inversion recovery (STIR; TR 4910, TE 40), and (c) contrast enhanced coronal T1 fat saturated(TR 540, TE 13) sequences showed a heterogeneous soft tissue mass at the insertion of pes anserinus conjoint tendon with peripheral and septal enhancement. The decreased interspersed signal on all images was due to hemosiderin deposits. There was no evidence of bone or joint involvement. induration, bruit, dilated vein, or signiﬁcant regional lym- phadenopathy. It was noncompressible with no change in size on limb elevation. Tinel’s sign was negative, and distal neurovascular examination was unremarkable. There was full active painless range of motion at the knee without any instability or meniscal signs. The rest of her skeletal system examination was unremarkable. Radiographs were unre- markable. The magnetic resonance imaging (MRI) showed a 4.5 × 3.0 × 2.0 cm soft tissue mass at the insertion of pes anserinus conjoint tendon (Figures 1(a)–1(c)). There were areas of low to intermediate signals in all sequences, and the mass enhanced heterogeneously with contrast. There was no evidence of bone or joint involvement. Laboratory test results were noncontributory, including her coagulation proﬁle. An incisional biopsy was performed. The frozen sections showed it to be benign with multiple osteoclast like mult- inucleated giant cells in a mononuclear background, and a intralesional resection was performed. The specimen con- sisted of a brownish-yellowish lobulated multinodular mass in the pes anserinus bursa and measured 4.5 × 3.0 × 1.8 cm (Figures 2(a)–2(b)). The knee joint was not involved. Part of the semitendinosus tendon had to be sacriﬁced because of direct involvement of the lesion, and the remnants were ten- odesed to the rest of the pes anserinus tendons. Histopathol- ogy showed osteoclast-like multinucleated giant cells in a mononuclear background with foci of hemosiderin deposits (Figures 3(a)-3(b)). There was no atypia, mitosis, or necrosis, and the surrounding soft tissue appeared unremarkable. A clinical diagnosis of PVNB of pes anserinus was made. The postoperative course was uneventful, and the patient is free of disease with no functional deﬁcit at 2 years followup. 3. Discussion Pigmented villonodular synovitis is a benign but potentially aggressive lesion, characterized by synovial villonodular proliferation with hemosiderin pigmentation and stromal inﬁltration of histiocytes and giant cells. Although it was sug- gested an inﬂammatory process [1], the insigniﬁcant degree of inﬂammation, the nodular growth pattern, propensity for local recurrence, distinct lack of changes characteristic of the lesion in the adjacent synovial tissue, and DNA ﬂow