Milea Timbergen

161 Introduction Desmoid-type fibromatosis (DTF) is an uncommon, soft-tissue tumour arising in musculoaponeurotic structures and mainly affecting young adults aged between 20 and 40 years 1 . DTF is characterised by unpredictable, invasive growth. Rapid growth is often seen in the early phase of the disease, but also in response to pregnancy or hormonal manipulation 2, 3 . After an initial period of growth, many patients experience prolonged stabilisation of the desmoid tumour. Up to ten years ago, surgical treatment was the mainstay of treating DTF leading to significant morbidity and high recurrence rates 4-6 . Other forms of active treatments, such as radiotherapy and systemic therapy, mainly have a role in case of progressive and symptomatic tumours located at sites which are difficult to treat surgically 7 . However, these therapies can lead to treatment-related toxicities 7 . The term ‘active surveillance’ (AS) for the management of DTF was introduced in the 1990s. Initially, AS was only offered to patients with recurrent tumours, but after 2005 also patients with primary tumours were exposed to this approach 8, 9 . As a results, a decrease in the use of these ‘active treatments’ over the past years has been reported in several nation-wide cohort studies 4, 5 . AS for DTF is justified as it has no metastatic potential and spontaneous tumour regression is reported in up to 30% of patients who undergo initial AS 10 . A large retrospective study showed no difference in event-free survival (EFS) comparing surgery with the AS approach (53% versus 58%, p = 0.415) 6 . The first European consensus guideline dates from 2015, and advocates using AS as an upfront approach, to minimise overtreatment and to prevent unnecessary morbidity 11 . This recommendation was based on the results of several retrospective series 8, 10, 12-14 . A systematic review to summarise and to evaluate the published results of the AS approach can be helpful to select patients who benefit from this approach, while awaiting the results of three ongoing, prospective clinical trials from Europe (NCT01801176, NCT02547831, and NTR 4714) 15-17 . The aim of the current study was to systematically review published studies reporting the results of the AS approach in adult DTF patients. Furthermore, Response Evaluation Criteria in Solid Tumors, (RECIST) classification of DTF tumours during the AS approach was evaluated, prognostic factors for a successful AS approach were identified, the median time to shift to an active form of treatment and the median duration of the AS approach were analysed and lastly, the forms of active treatment after the initial AS approach were assessed. 6