Case Reports in Medicine multitude of nonhemosiderin-ﬁlled macrophages that typify PVNS/PVNB [7]. A hemangioma may have calciﬁed phle- boliths and is morphologically homogenous with distinct histology [5, 6, 12]. A concomitant papillary synovium, with multiple telangiectasias mimicking intrasynovial heman- gioma has been described by Kay et al. [7]. Histopatho- logically, the pronounced cellularity with a possibility of locally destructive mass can mislead to a diagnosis of malignancy [9, 14]. The geographical pattern of xanthoma- tous regions alternating with cellular hyalinized regions contrasts with the more uniform spindled appearance of synovial sarcoma and the primitive round cells of childhood rhabdomyosarcoma [14]. Moreover, cellular pleomorphism and mitotic activity goes in favor of a malignancy. Synovial sarcoma, usually, has biphasic morphology, may contain mucin, but shows no gross villi, nodules, or pigmen- tation. Moreover, about one-third of them may show mineralization. Diﬀuse giant cell tumors with prominent xanthomatous component must also be diﬀerentiated from inﬂammatory or xanthomatous forms of malignant ﬁbrous histiocytoma or undiﬀerentiated high-grade pleomorphic sarcoma [14]. The latter usually occur in retroperitoneum and contain xanthomatous areas and spindled areas resem- bling the conventional form. Moreover, these have pre- dominant acute inﬂammatory background, which contrasts with the modest number of chronic inﬂammatory cells in giant cell tumors. Focal necrosis may also be present in PVNS due to torsion of a pedunculated nodule. The synovium-based location and apparent maturation at the periphery may be helpful in diagnosis. Because of this wide spectrum of diﬀerential diagnosis, a histologic diagnosis be- comes essential before planning the deﬁnite surgical proce- dure (intralesional, marginal, or wide local resection). Although much has been written about behavior and treatment of PVNS, the literature is sparse on PVNB. Recur- rence has been reported to be as high as 40–50% for diﬀuse PVNS and most of them have been correlated with a location in the knee and incomplete excision [14]. Morphology has not been found to be predictive of recurrence. Although the followup has been limited on most of the reported cases (Table 1), only one had recurred 13 years later. This could be attributed to the localized and extra-articular nature of pes anserinus PVNB, which allows the surgeon to be more aggressive and completely resect the mass without fear of signiﬁcant functional loss. 4. Conclusion Extra-articular PVNB of the pes anserinus is a rare cause of medial knee pain and localized swelling about the proximal tibia. Awareness of this rare presentation is important because it will help in correct diagnosis and avoid overtreat- ment. 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