To assess the feasibility, safety and preliminary efficacy of magnetic resonance guided focused ultrasound (MRgFUS) for the treatment of extra-abdominal desmoid tumors.Institutional review board approval was obtained for this HIPAA-compliant retrospective study. Patients or their representatives provided informed consent before treatment. From March 2011 to January 2015, fifteen patients (6 males, 9 females, mean age: 28 years) with desmoid fibromatosis were treated at four institutions with MRgFUS (25 total treatments) (Figure 1a - e). Changes in total and viable tumor volumes after treatment were measured. Efficacy was defined by the median reduction in viable tumor exceeding a threshold of 50% of the targeted volume. Median decrease after treatment of at least two points in numerical rating scale (NRS) worst and average pain scores was tested. An exact one-sided Wilcoxon test was used for statistical analyses. Adverse events were recorded.Tumor volumes ranged from 4 – 1010 mL. Total tumor volume stabilized or decreased after the treatments (Figure 2a). Viable total tumor volume decreased from 212 ± 286 mL to 96 ± 126 mL after initial treatment (Figure 2b). The median reduction of 63% of the viable targeted tumor volume was significantly beyond our efficacy threshold (P = 0.0013). Pain improved after treatment (worst scores: 7.5 ± 1.9 vs. 2.7 ± 2.6,P = 0.027; average scores: 6 ± 2.3 vs. 1.3 ± 2, P = 0.021). Skin burn was the most common complication.Desmoid tumors are benign, but are locally infiltrative and may cause pain and dysfunction. Standard therapies using surgical resection, radiation and systemic chemotherapy suffer from inadequate efficacy and excessive side effects for treatment of a benign disease¹. This multicenter study demonstrates that MRgFUS treatment of desmoids is feasible and that this technique may be used to control the growth of symptomatic desmoid tumors of the extremities, with decrease in both total and viable tumor volumes. In our fifteen patients there was a 63% median reduction in perfused volume of the targeted tumors immediately after treatment; this surpassed our target for feasibility, which was at least 50% of tumor ablation. Pain related to the tumors also improved, with a 64% decline in maximum pain scores after tumor ablation. There does not appear to be a cumulative toxicity limit to surrounding normal structures, allowing re-treatment as necessary to ablate residual or recurrent tumor. Treatment with MRgFUS does not appear to accelerate the growth of the tumor, which was a theoretical concern given the possibility that trauma or surgery may lead to tumor progression². MRgFUS treatment was feasible after surgery or chemoradiation, and was also combined successfully with other treatment modalities, such as chemotherapy and cryoablation, to treat remaining tumor.This series of 15 patients treated at multiple institutions demonstrates that MR-guided focused ultrasound can achieve durable local control of extra-abdominal desmoid tumors. Because it is non-invasive and repeatable, MRgFUS may provide a useful way to treat desmoid tumors while avoiding the morbidities of surgery, radiation and systemic chemotherapy.