Milea Timbergen

126 This study investigated whether a radiomics model based on MRI is able to 1) distinguish DTF from non-DTF in the extremities, and 2) to predict the CTNNB1 mutation status of DTF. Additionally, in the DTF versus non-DTF distinction, we evaluated which of the included MRI sequences has the highest predictive value. Material and methods Data collection Approval by the Erasmus Medical Center (MC) institutional review board (MEC-2016-339) was obtained. Patients diagnosed or referred to the Erasmus MC between 1990-2018 with a histologically proven primary or recurrent DTF were included. This resulted in a multicentre imaging dataset as patients referred to our sarcoma expert institute often received imaging at their referring hospital. The most frequently used imaging modality prior to treatment was T1w-MRI, and its availability was used as an inclusion criterion 18 . When available, other sequences such as T2w, T1w post-contrast, dynamical contrast enhanced (DCE), proton density (PD) and diffusion weighted imaging (DWI) MRI were collected. For the differential diagnosis (DTF versus non-DTF), histologically confirmed malignant extremity STS were included. Benign STS were excluded, because this distinction is clinically less relevant. Non-extremity STS were excluded because of the infrequent use of MRI. Although DTF tumours commonly occur in the abdominal wall, their differential diagnosis is broad and includes pseudo-tumours such as myositis, nodular fasciitis and hematomas, and tumours such as lipomas, STS, endometriosis, carcinomas, lymphomas and metastasis 19 . Hence, we decided to focus on the distinction between DTF and STS, and included patients with a histologically proven primary fibromyxosarcoma, myxoid liposarcoma or leiomyosarcoma of the extremities. Similar to the DTF, patients with at least a pre-treatment T1w-MRI were retrospectively included. Sex, age at diagnosis, and tumour site were collected. For the DTF, in case of a missing CTNNB1 mutation status, Sanger Sequencing was performed after review of formalin-fixed paraffin-embedded tumour sections by a pathologist. Cases with a known CTNNB1 mutation did not undergo additional review by a pathologist. Poor scan quality (e.g., artefacts), poor DTF DNA quality with failure of sequencing, and CTNNB1 mutation other than S45F, T41A or WT led to exclusion. 5