Colorectal peritoneal metastasis incidence and survival in the United States: A SEER retrospective cohort study

BackgroundThere are limited data on the incidence, distribution, and prognosis of colorectal peritoneal metastasis. However, some studies have suggested that colorectal peritoneal metastasis has a worse prognosis that other sites of metastasis. Therefore, this study assessed the epidemiology and prognosis of colorectal metastasis categories in the United States. MethodsThe incidence-based SEER database was used to identify patients with metastatic colorectal adenocarcinoma diagnosed from 2018-2021. Extent of disease and staging variables were used to determine sites of metastasis. Demographic and treatment variables were included as covariates. Descriptive statistics and survival analyses were conducted. ResultsThe cohort consisted of 12,117 patients and distribution by staging category was M1a (46%), M1b (26%), M1c (19%), and uncategorized (9%). Among patients with M1c disease, 46.7% had peritoneal-only metastasis, comprising 9% of the entire cohort. The M1c cohort had a higher proportion of right-sided tumors (45.7%) and mucinous histology (10%) compared to M1a (33% and 2%, respectively) or M1b (30% and 2%, respectively). Median overall survival for M1a, M1b, and M1c was 20, 11, and 11 months, respectively (p<0.05). In adjusted analysis, the restricted mean survival time was 21.6, 17.1, and 17.5 months for M1a, M1b, and M1c cohorts, respectively and for M1c isolated and multisite metastasis was 20.3 and 14.7 months, respectively (p:<0.001). ConclusionsColorectal peritoneal metastasis accounted for approximately 20% of synchronous metastatic colorectal cancer cases. Although the absolute survival differences between M1b and M1c disease were negligible, there were significant survival differences between isolated M1c peritoneal-only disease compared M1c multisite (peritoneal and hematogenous) metastasis. These data demonstrate that colorectal peritoneal metastasis is not rare and the survival differences in the M1c subgroup have implications for clinical staging and suggest unique tumor biology.