Milea Timbergen

192 variable. An interaction effect between tumour size and CTNNB1 mutation type was tested in the multivariable Cox model adjusting for age, sex, mutation type, tumour site and tumour size; subgroup analyses based on tumour size were considered in case of considerable interaction. Two-sided p < 0.05 was considered statistically significant. SPSS Statistics (version 24) was used for all statistical analyses (IBM, Armonk, New York, USA). Results Study selection and study characteristics Results of the search strategy are presented in the flowchart provided in Supplemental Table 2. A total of 47 articles were screened based on full text for eligibility. Studies were excluded for the following reasons: describing a pediatric cohort (n = 1), no CTNNB1 mutation type data available (n = 14), no full text available (n = 3), conference abstract (n = 1), no recurrence data described (n = 6), no DTF as main subject (n = 1) and a review article (n = 3). Based on the origin of the patient cohort, eight studies had to be excluded for describing patient sets having a large overlap with series already published 19, 24, 25, 29-33 . The papers describing the largest cohort were included to request the IPD. From others we were not able to receive the IPD 15, 34, 35 . All included articles concerned retrospective cohort studies and received a score of 2b at the Oxford Levels of Evidence 2011 27 . The QUIPS tool was used for assessing the risk of bias (Supplemental Table 4) 28 . A detailed description of the published reports of the included studies can be found in Supplemental Table 5. Individual patient data and clinical characteristics A total of ten corresponding authors were contacted for exchanging IPD. Seven out of ten authors were willing to share the data. Data from individual patients were provided in a cohort template (n = 1), a database containing a selection of patients (n = 2) or by sharing the entire database (n = 4), and the cohorts were analysed by one author (M.J.M.T.). Patients were screened using the aforementioned inclusion and exclusion criteria for IPD, leaving a total number of surgically treated adult patients with primary DTF tumours of 329. The majority of the patients were female (n = 247, 75.1%) with a median age of 38 years (IQR: 31-50 years), and the most common tumour site was the trunk/back (n = 194, 59%). The majority (n = 154, 46.8%) of patients had a CTNNB1 T41A mutated DTF tumour. Other clinical characteristics are summarized in Table 1. Missing values included unknown resection margin status for 18 (5.5%) patients, missing tumour size (in mm) for 7 patients (2.1%), and an extra-abdominal tumour site which was not further specified for 49 patients (14.9%). The latter was included as separate category for the variable tumour site. 7