Differences in overall survival and mutation prevalence between right- and left-sided colorectal adenocarcinoma

Prior reports have demonstrated inferior outcomes for patients with right-sided colorectal cancer (CRC) compared to patients with left-sided disease, as well as differences in treatment response based on disease sidedness. Differences in prognosis remain even among patients with metastatic disease, indicating that anatomy or stage at diagnosis alone cannot explain all of these findings. While genetic differences between right- and left-sided CRC have long been described, the genetic and molecular drivers underlying differences in prognosis and treatment response remain incompletely understood.

We compared mutation prevalence between right- (cecum to splenic flexure) and left-sided (descending colon to rectum) CRC among 38 genes in a retrospective review of next-generation sequencing data of CRC samples obtained in routine clinical practice at a single academic medical center.

Among 288 cases (167 left-sided, 103 right-sided, 18 synchronous or without clear primary), patients with left-sided primaries had a longer overall survival from pathologic diagnosis (median 1,823 days vs. 1,006 days for right-sided cases, P=0.004). Among the assessed genes, BRAF and CTNNB1 mutations were more prevalent in right-sided CRC. BRAF was mutated in 15.5% of right-sided CRC (95% CI: 8.5-22.5%) compared to 4.8% (95% CI: 1.6-8.0%) (P=0.003). CTNNB1 was mutated in 3.9% of right-sided CRC (95% CI: 0.2-7.6%) compared to no instances of CTNNB1 mutations in left-sided disease (P=0.01).

This difference in mutation prevalence may implicate these genetic pathways in the mechanisms underlying the discrepant outcomes and treatment responses between right- and left-sided CRC described in this and prior studies.