289 Predictive genetic biomarkers for the development of peritoneal metastases in colorectal cancer small amounts of DNA and RNA as input. Combining DNA and RNA hybrid-capture with sophisticated informatics reduces errors and yields high-quality data, even from FFPE samples. We did not perform an extensive sample size calculation due to the predictive and explorative character of this study. Despite the efforts made to create as much homogeneity between the three groups as possible, the number of patients in our cohort is small. A larger-scale study should be conducted to confirm the mutation differences in relation to PMs. Thereby, being a retrospective study, there is a likelihood of selection bias and information bias. Additionally, we performed a very broad cancer gene analysis with our TSO500 panel, although the method does not cover all genes. Through performing whole exome or genome sequencing (WES or WGS), potential candidate genes that can act as a predictive PM biomarker that are not included in the TSO500 panel may be identified. Unfortunately, WES and/or WGS are more expensive and have additional logistic limitations. CONCLUSION Over the last decade, the genetic analysis of CRC has evolved enormously, resulting in better tumor classifications, improved treatment decisions, and finally enabling personalized treatment options. Specific genetic changes and mutations that could predict PM remain largely unknown. In our cohort, we identified genes that have not been described in relation to metachronous PMs, or metastases in general, before. The clinical significance of this finding remains unknown due to the small sample size. BRAF V600E mutations were only present in PM patients with MSS tumors. We believe greater attention should be paid to BRAF-mutated tumors in relation to the development of metachronous PMs. Future prospective research into and validation of the molecular players identified here, specifically within non-synchronous tumors, might influence the efficacy of existing and future diagnostic (biomarker identification), prognostic (patient grouping and recurrence), and therapeutic (molecular) actions. ACKNOWLEDGEMENTS AND STATEMENTS Bjorn Winkens was involved as a professional statistician to ensure an appropriate statistical analysis. We would like to thank all analysts from the pathology department for contributing to this project. This work was supported by the Stichting Jules Coenegracht Sr. RSIN 8148.99.110. The study was conducted according to the guidelines of the Declaration of Helsinki and was approved by the Institutional Medical Ethics Committee from MUMC+ (nr. 2021-2888) and CZE (nr. 2021-089). 12
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