Milea Timbergen

125 Introduction Sporadic desmoid-type fibromatosis (DTF) is a rare borderline, soft tissue tumour arising in musculoaponeurotic structures 1 . Worldwide epidemiological data is lacking, but population studies in Scandinavia and the Netherlands show a low incidence of 2.4–5.4 cases per million per year 2, 3 . Early recognition and diagnosis of DTF is therefore challenging. On MRI, DTF can display a wide variety of enhancement patterns 4 . DTF has imaging characteristics that are often associated with soft tissue sarcomas (STS), such as crossing fascial boundaries, an invasive growth pattern, little central necrosis, mildly hyperintense on T1-weighted (T1w) MRI, and hyperintense and heterogeneous on T2-weighted (T2w) MRI with hypointense bands 5 . Hence, the distinction between DTF and STS, i.e., non-DTF, can be difficult. An invasive tissue biopsy, with additional immunohistochemical staining for β-catenin and mutation analysis of the CTNNB1 (β-catenin) gene, is therefore currently required to differentiate DTF from non-DTF 6 . As DTF is a borderline tumour who is unable to metastasize, and requires a different treatment regimen than malignant STS, this distinction is highly relevant. Differentiation between DTF and STS based on imaging would be beneficial because of the rarity of DTF, making clinical and pathological recognition challenging. Furthermore, DTF exhibits an aggressive growth pattern and growth might be stimulated after (surgical) trauma, including biopsies 7 . Avoiding (multiple) harmful biopsies which potentially cause tumour growth is therefore of great importance. Several studies have addressed the prognostic role of the CTNNB1 mutation in DTF 8-10 , as serine 45 (S45F) tumours appear to have a higher risk of recurrence after surgery compared to threonine 41 (T41A) and wild-type (WT) (i.e. no CTNNB1 mutation 11 ) tumours 12 . Obtaining the CTNNB1 mutation status is for diagnostic purposes and to guide the clinical work-up, but, for now, the CTNNB1 mutation status has no therapeutic consequences 13 . The majority of DTF harbours a CTNNB1 mutation at either T41A or S45F 8 . Assessment of the mutation status is currently done by Sanger Sequencing or Next Generation Sequencing, which are time consuming and expensive. In radiomics, large amounts of quantitative imaging features are related to clinical outcome 14 . Radiomics may serve as a non-invasive surrogate to contribute to diagnosis, prognosis and treatment planning 15, 16 . Based on the results of previous studies in cancer 17 , we hypothesized that radiomics may also be useful in DTF. 5