Abstract 【Objective】 To evaluate the clinical efficacy of arthroscopy combined with open-posterior synovectomy and adjuvant low-dose external radiotherapy for the treatment of diffuse pigmented villonodular synovitis (DPVNS) of the knee.【method】A retrospective study was performed on 52 patients with diffuse PVNS of the knee who underwent synovectomy in our hospital from January 2010 to December 2019. They were divided into four groups based on treatment, 8 patients were treated with arthroscopic synovectomy (group I), 22 patients were treated with arthroscopic synovectomy, and adjuvant postoperative radiotherapy (group II), 5 patients were treated with arthroscopy combined with open-posterior synovectomy (group III),17 patients were treated with arthroscopy combined with open-posterior synovectomy and adjuvant radiotherapy (group IV). The changes of Ogilvie-Harris score and postoperative recurrence rate of the four groups were observed. 【Results】52 patients were followed up: 59.83?20.19 months (25-118), the recurrence rate of group I: 37.50%, the recurrence rate of group II: 4.55%, the recurrence rate of group II was lower than that of group I (P<0.05), the recurrence rate of group III: 60%, the recurrence rate of group IV: 5.88%, the recurrence rate of group IV was lower than that of group III (P<0.05), there was no difference between the recurrence rate of group II and group IV (P>0.05). The Ogilvie-Harris scores of the four groups were significantly improved at the last follow-up compared with those before surgery (P<0.05). 【Conclusion】The arthroscopic surgery is recommended for diffuse PVNS limited to intra-articular synovium, the arthroscopy combined with open-posterior synovectomy is recommended for diffuse PVNS spreading to extra-articular tissues. The recurrence rates were reduced by adjuvant low dose radiotherapy in both of them, and there is no significant difference in the recurrence rate between the two groups after adjuvant radiotherapy.