Background: When treated with molecular targeted agents, patients with unresectable colorectal cancer with right-sided tumors have poorer prognoses than those with left-sided tumors. While primary tumor sidedness may have prognostic value, the prognostic value of tumor sidedness in chemotherapy regimens without targeted therapy is unclear.
Methods: Our study population comprised 678 consecutive patients with unresectable stage IV colorectal cancer who received systemic chemotherapy at the National Cancer Center Hospital in Japan from 1999 to 2015. Patients were stratified by treatment subgroup (with or without molecular targeted agents and with or without palliative primary tumor resection), and relationships between overall survival (OS) and primary tumor sidedness were evaluated. Multivariate analyses were also performed.
Results: Overall, 193 (28%) tumors were right-sided (cecum to transverse colon) and 485 (72%) were left-sided (splenic flexure to rectum). In the overall population, median survival time was 16.4 months for those with right-sided tumors and 23.4 months for those with left-sided tumors (p < 0.01). Regardless of the use or non-use of targeted agents and performance or non-performance of palliative resection of the primary tumor, those with right-sided tumors showed significantly poorer prognosis than those with left-sided tumors, in all categories. Multivariate analyses showed right-sided tumors to be associated with shorter OS compared with left-sided tumors (hazard ratio 1.26, 95% confidence interval 1.03-1.53; p =0.024).
Conclusion: Unresectable stage IV right-sided colorectal tumors were associated with shorter OS compared with left-sided tumors, regardless of treatment strategy. Primary tumor sidedness may be an independent prognostic factor.