Association of Cancer—Associated Venous Thromboembolism with the Primary Site of Colorectal Cancer, with Respect to KRAS/NRAS/BRAF Mutations

<b>Background/Objectives:</b> Venous thromboembolism (VTE) is a common and clinically significant complication in patients with metastatic colorectal cancer (mCRC). Tumor sidedness and molecular alterations such as RAS and BRAF mutations are established prognostic factors in mCRC; however, their role in VTE risk stratification remains unclear. This study aimed to evaluate the association between primary tumor sidedness, KRAS/NRAS/BRAF mutational status, and VTE occurrence in patients with mCRC treated in the outpatient setting. <b>Methods</b>: This multicenter ambispective observational study included 224 patients with mCRC treated with first-line chemotherapy with or without targeted therapy. All patients had known KRAS/NRAS/BRAF statuses. The primary endpoint was the association between tumor sidedness and VTE risk. Secondary endpoints included associations between oncogenic mutations and VTE, subgroup analyses according to tumor localization and mutational status, and overall survival (OS). Multivariate logistic regression was used to identify independent predictors of VTE. <b>Results</b>: After a median follow-up of 21 months, VTE occurred in 23.3% of patients. The incidence of VTE was significantly higher in right-sided colorectal cancer (RCRC) compared with left-sided colorectal cancer (LCRC) (41.0% vs. 17.6%, <i>p</i> < 0.001). Although KRAS/NRAS and BRAF mutations were more frequent in RCRC, mutational status was not independently associated with VTE. In multivariate analysis, right-sided tumor location remained a strong predictor of VTE (OR 5.2; 95% CI 1.9-14.1; <i>p</i> = 0.001), along with anti-EGFR therapy. The Khorana score classified most patients as low risk and did not reliably identify those who developed VTE. VTE occurrence was not significantly associated with OS, whereas right-sided tumor location was associated with inferior survival. <b>Conclusions</b>: Right-sided tumor location is an independent predictor of VTE in patients with mCRC and confers a high absolute thrombotic risk not captured by the Khorana score. Incorporating tumor sidedness into VTE risk assessment may improve identification of patients who could benefit from primary thromboprophylaxis.