Milea Timbergen

160 Abstract Background This study evaluates the results of the active surveillance (AS) approach in adult patients with desmoid-type fibromatosis (DTF), because AS is advocated as a front-line approach for DTF in the European consensus guidelines. Materials and methods A systematic literature search was conducted (December 19th, 2019, updated on April 14th, 2020). Studies describing the outcomes of the AS approach were included. The PRISMA guidelines were used. Results Twenty-five articles were included for data-retrieval. Forty-two percent of reported patients (1480 of 3527 patients) received AS, the majority were females and the majority had a primary tumour. The median age at diagnosis ranged from 28 to59 years. Common tumour sites were; the extremities/girdles (n = 273), the abdominal wall (n = 253), and the trunk (n = 153). The median reported percentage of progressive disease, stable disease and partial response was 20% (interquartile range [IQR] 13-35%), 59% (IQR: 37-69%) and 19% (IQR: 3-23%), respectively. In 640 patients, the outcome was not specified. The median reported percentage of shifting to an active form of treatment was 29%, most commonly to systemic treatment (n=195) and surgery (n=107). The reported median follow-up time ranged between 8 and 73 months. The reported median time to progression and/or initiation of the subgroup shifting from AS to ‘active’ therapy ranged from 6.3 months to 19.7 months. Conclusions The majority of patients undergoing AS have either stable disease or a partial response, and about one-third of patients shift to an active form of treatment. Selecting patients who will benefit from active surveillance upfront should be the priority of future studies. 6