Baseline Characteristics
A total of 60358 stage IV CRC patients were identified. 34177 patients received primary tumor resection while 26181 patients did not receive primary tumor resection. If the patients with surgery of metastases were excluded, 25126 patients received palliative primary tumor resection while 24750 patients did not receive palliative primary tumor resection. The data about site of metastases were only available between 2010-2015. There were 302, 107, 16501, and 1918 patients with isolated bone, brain, liver and lung metastases, respectively. There were 18828, 5720, 707, and 41 patients with single, double, triple, and quadruple metastatic sites, respectively.
Prognostic factors of cancer-specific survival
Based on univariate analysis, age at diagnosis, gender, marital status, race, serum CEA, histologic type, differentiation tumor location, surgery of primary lesion, surgery of metastases, site of metastases, chemotherapy and radiotherapy affected significantly cancer-specific survival. When these factors were entered a multivariate analysis, it showed that age at diagnosis, marital status, race, serum CEA, histologic type, differentiation tumor location, surgery of primary lesion, surgery of metastases, site of metastases, chemotherapy and radiotherapy were significantly associated with cancer-specific survival (Table 1).
Subgroup of metastatic CRC
We next divided the patients into different subgroups to evaluate the value of primary tumor resection according to the independent prognostic factors. Univariable Cox regression analysis revealed that patients under 60 years old benefited more from primary tumor resection as compared to those over 60 years old (≤60 years old: HR, 0.454; 95% CI, 0.440-0.468; P<0.001, >60 years old: HR, 0.500; 95% CI, 0.488-0.512, P<0.001). Besides that, patients with non-white people (white: HR, 0.489; 95% CI, 0.478-0.500; P<0.001, black: HR, 0.447; 95% CI, 0.426-0.469, P<0.001), normal serum CEA (normal serum CEA: HR, 0.445; 95% CI, 0.418-0.473; P<0.001, elevated serum CEA: HR, 0.498; 95% CI, 0.485-0.510, P<0.001), well or moderate differentiation (well differentiation: HR, 0.483; 95% CI, 0.437-0.534; P<0.001, moderate differentiation: HR, 0.482; 95% CI, 0.469-0.496, P<0.001, poor differentiation: HR, 0.558; 95% CI, 0.535-0.582; P<0.001, undifferentiated differentiation: HR, 0.569; 95% CI, 0.492-0.658, <0.001), surgery of metastases (no surgery: HR, 0.514; 95% CI, 0.504-0.525; P<0.001, surgery: HR, 0.456; 95% CI, 0.427-0.487, P<0.001), isolated liver metastases (lung only: HR, 0.484; 95% CI, 0.427-0.549; P<0.001, liver only: HR, 0.453; 95% CI, 0.435-0.471, P<0.001, brain only: HR, 0.717; 95% CI, 0.459-1.120; P=0.144, bone only: HR, 0.665; 95% CI, 0.491-0.899, P=0.008), single metastatic site (single site: HR, 0.463; 95% CI, 0.446-0.481; P<0.001, double sites: HR, 0.643; 95% CI, 0.601-0.688, P<0.001, triple sites: HR, 0.631; 95% CI, 0.512-0.777; P<0.001, quadruple sites: HR, 0.913; 95% CI, 0.376-2.215, P=0.840), chemotherapy (no chemotherapy or unknown: HR, 0.520; 95% CI, 0.505-0.535; P<0.001, chemotherapy: HR, 0.457; 95% CI, 0.446-0.469, P<0.001) and radiotherapy (no radiotherapy: HR, 0.472; 95% CI, 0.463-0.482; P<0.001, radiotherapy: HR, 0.414; 95% CI, 0.391-0.438, P<0.001) gained more survival benefits. Of note, patients with signet ring cell carcinoma benefited less from primary tumor resection (adenocarcinoma: HR, 0.472; 95% CI, 0.462-0.482; P<0.001, mucinous adenocarcinoma: HR, 0.484; 95% CI, 0.451-0.519, P<0.001, signet ring cell carcinoma: HR, 0.623; 95% CI, 0.550-0.707; P<0.001). No significant difference in survival benefit was observed between patients with different marital status (unmarried: HR, 0.493; 95% CI, 0.479-0.507; P<0.001, married: HR, 0.476; 95% CI, 0.464-0.490, P<0.001) or different tumor location (colon: HR, 0.436; 95% CI, 0.426-0.446; P<0.001, rectum: HR, 0.452; 95% CI, 0.435-0.470, P<0.001). Similar results were observed in multivariable Cox regression analysis except for age (Table 2). To evaluate the value of palliative resection of primary tumor, patients who underwent surgery of metastases were excluded from the analysis. Intriguingly, subgroups that benefited more from palliative resection of primary tumor were consistent with those in entire cohort (Table 3).