A Prospective Observational Study of Active Surveillance in Primary Desmoid Fibromatosis

Purpose: To prospectively assess the behavior of primary spo-radic (not familial adenomatous polyposis-associated) desmoid fibromatosis (DF) managed by active surveillance (AS). Experimental Design: This is an Italian prospective, multicenter, observational study (NCT 02547831) including patients >= 16 years with primary sporadic DF at any site. Patients were assessed by Response Evaluation Criteria in Solid Tumor (RECIST) version 1.1. Primary endpoint was progression-free survival (PFS) at 3 years. Treatment-free survival (TFS) was also analyzed. PFS and TFS were calculated by Kaplan-Meier plots and compared by log-rank test. Cox proportional hazard multivariable regression analyses were performed. Results: From 2013 to 2018, 108 consecutive patients were included (82% female); median age was 39 years; median size was 51 mm. CTNNB1 mutations were T41A (50%), S45F (12%), other (19%), wild-type (19%). At 32.3-month median follow-up, 42 of 108 (39%) showed RECIST progression. Spontaneous regression was initially observed in 27 of 108 (25%), while it followed dimensional progression in another 33 of 108 (31%). PFS at 36 months was 54.5% [95% confidence interval (CI), 44.9%-66.1%]. Thirty-five of 108 (32%) patients received active treatment, 18 of 108 (17%) after RECIST progression and 17 of 108 (15%) after symptomatic progression. TFS at 36 months was 65.9% (95% CI, 57.3%-75.9%). Larger tumor size and extremity location were associated to shorter TFS and a trend for S45F mutation was also observed (P 1/4 0.06), while none of the mentioned variables was significantly associated with PFS. Conclusions: In primary DF, AS can be proposed, because disease stabilization and spontaneous regression frequently occur. However, extra care should be taken for patients with tumors of larger size, extremity location, and S45F mutation. See related commentary by Greene and Van Tine, p. 3911