Time‐varying prognostic value of primary tumor sidedness in metastatic colorectal cancer: A population‐based study and meta‐analysis

We studied the prognostic value of primary tumor sidedness in metastatic colorectal cancer over time and across treatment lines. Population data on synchronous metastatic colorectal cancer patients were extracted from the Netherlands Cancer Registry and SEER database. Pubmed, EMBASE and Cochrane library were searched for prospective studies on metastatic colorectal cancer to conduct a meta‐analysis. Inclusion criteria consisted of metastatic disease, systemic treatment with palliative intent and specification of primary tumor location. Data were pooled using a random‐effects model. For the population‐based data, multivariable Cox models were constructed. The Grambsch‐Therneau test was conducted to evaluate the potential time‐varying nature of sidedness. Meta‐regression incorporating treatment‐line as variable was conducted to test the pre‐specified hypothesis that the prognostic value of sidedness varies over time. Analysis of 12 885 and 16 160 synchronous metastatic colorectal cancer patients registered in the Netherlands Cancer Registry and SEER database, respectively, indicated a time‐varying prognostic value of sidedness (P < .01). Thirty‐one studies were selected for the meta‐analysis (9558 patients for overall survival analysis). Pooled univariable hazard ratio(left‐sided/right‐sided) for overall survival was 0.71 (95% CI: 0.65‐0.76) in 1st‐line, 0.76 (0.54‐1.06) in 2nd‐line and 1.01 (0.86‐1.19) in 3rd‐line studies. Hazard ratios were significantly influenced by treatment line (P = .035). The prognostic value of sidedness of the primary tumor in metastatic colorectal cancer patients treated with palliative systemic therapy decreases over time since diagnosis, suggesting that sidedness may not be a useful stratification factor in late‐line trials. This decrease in prognostic value should be taken into account when providing prognostic information to patients.