Pigmented villonodular synovitis (PVNS) is a rare proliferative disorder that affects the synovium in young and middle-aged adults. Although most believe that it is an inflammatory process, some believe that it is a benign neoplasm. The optimal treatment is surgery. The local recurrence rate after marginal excision for localized PVNS is low. In contrast, the local recurrence rate after open synovectomy for diffuse PVNS is relatively high. The intra-articular instillation of radioactive isotopes or external beam radiotherapy (approximately 35 Gy in 14-15 fractions over 3 weeks) may significantly improve the likelihood of local control and long-term function in patients with incompletely resected diffuse PVNS. The probability of complications after moderate-dose radiotherapy (RT) is low.