Milea Timbergen

189 Study selection Two independent review authors (M.J.M.T. and M.R.) assessed the retrieved articles of the search for potential inclusion by reviewing title and abstract. Next, full articles were evaluated according to the predetermined inclusion and exclusion criteria for this meta- analysis (listed in Supplemental Table 3). Assessment of study quality and risk of bias The Oxford levels of Evidence (Oxford Centre for Evidence Based Medicine) were used to assess the quality of included articles 27 . The quality of included studies was assessed using the Quality In Prognostic Studies (QUIPS) tool 28 . This tool consists of six domains of potential biases: study participation, study attrition, prognostic factor measurement, outcome measurement, study confounding, statistical analysis and reporting, rated as “low”, “moderate”, or “high” risk of bias. When all domains were considered to be “low” or “moderate” risk of bias, the risk of bias was considered to be low. If one or more domains were rated as “high” risk of bias, it was considered to be a high risk of bias. Two authors scored the risk of bias (M.J.M.T. and M.R.) and resolved discrepancies by discussion. Data collection process Two authors (M.J.M.T. and M.R.) independently extracted clinical and genetic information from the full text using a predefined extraction sheet. Subsequent cross-checks were performed. Inconsistencies were discussed and resolved. Patient overlap was defined as “the description of a cohort from one institute with overlapping time periods”. In the latter case, the largest cohort was included and IPD was requested from the corresponding author. Data items For each eligible article, the corresponding authors were contacted via email to retrieve the IPD. They were asked to provide the data either in a template database or by providing their database. Requested variables included: sex, date of birth, date of diagnosis, age at diagnosis, tumour site (extra-abdominal, intra-abdominal, abdominal wall), tumour site specified, tumour size, treatment type (active surveillance, surgery, radiotherapy, systemic therapy, or combination therapy), date of surgery, resection margin status, presentation (primary or recurrent tumour), FAP status, CTNNB1 mutation (WT, T41A, S45F, S45P or other), recurrence (yes or no), manner of identifying the recurrent tumour (radiological and/ or pathological confirmation), date of recurrence on radiology and/or pathology, date of last follow-up, vital status (death from disease, death from other cause, alive without evidence of 7