Integrated Molecular Profiling of Colorectal Cancer by Tumor Location: Evidence from a Real-World Cohort with Primary and Metastatic Samples.

Colorectal cancer (CRC) shows significant molecular diversity influenced by tumor location. Right- and left-sided CRCs differ in terms of microsatellite instability (MSI), mutational burden, and actionable biomarkers. This study aimed to characterize the clinicopathological and molecular features of CRC stratified upon tumor location. A consecutive series of CRC cases was retrospectively analyzed. Tissue samples were obtained from primary tumors (71%) or metastatic lesions (29%). All cases were evaluated by histopathology, immunohistochemistry (IHC), and targeted next-generation sequencing (NGS). Tumor location was assigned based on the primary tumor (43 right-sided and 35 left-sided cases). Right-sided CRCs were more frequent in older patients and females and showed higher rates of deficient MMR (42% vs. 17%, p = 0.02), MSI-H (39% vs. 14%, p = 0.02), and high tumor mutational burden (TMB-high, ≥10 Mutations/Mb, 56% vs. 28%, p = 0.02). The most frequent pathogenic class 5 mutations were TP53 (65%), APC (49%), and KRAS (44%). APC was the most frequently mutated gene in both pathogenic (class 5) and likely pathogenic (class 4) categories, with class 5 variants more common in left-sided tumors and class 4 variants predominating in right-sided tumors. BRAF mutations showed a statistically significant trend toward higher frequency in right-sided tumors (p = 0.05). HER2/neu overexpression (3+) was seen in 15% of patients, exclusively in MSS left-sided tumors. PD-L1 expression (CPS ≥ 1) was detected in 20% of patients, irrespective of location, and pan-TRK IHC was negative in all cases. The 29% of samples derived from metastatic lesions were predominantly MSS/pMMR (87%). Tumor location in CRC correlates with distinct molecular patterns. Right-sided tumors are associated with dMMR, MSI-H, and higher TMB, while left-sided CRCs display more ERBB2 alterations and class 5 APC mutations. The results highlight the importance of integrating tumor location into personalized molecular diagnostics and therapeutic planning for CRC patients.