Overall risk and risk factors for metachronous peritoneal metastasis after colorectal cancer surgery: a nationwide cohort study

BACKGROUND: This study aimed to identify the cumulative incidence and risk factors of metachronous peritoneal metastasis (M‐PM) from colorectal cancer in patients who had intended curative treatment. METHODS: Patients with colorectal cancer were identified using the Danish Colorectal Cancer Group database for 2006–2015. The Danish Pathology Registry and the Danish National Patient Registry were used to identify M‐PM to 2017. Risk factors were estimated by multivariable absolute risk regression, treating death and other cancers as competing risks. Overall risk and risk differences (RDs) were estimated at 1, 3 and 5 years. RESULTS: In 22 586 patients with colorectal cancer, the overall risk of M‐PM was reported to be 0·9 (95 per cent c.i. 0·8 to 1·0) per cent at 1 year, 1·9 (1·8 to 2·1) per cent at 3 years and 2·2 (2·0 to 2·4) per cent at 5 years. Advanced tumour category ((y)pT4 versus (y)pT1) increased the RD of both M‐PM (2·9 (95 per cent c.i. 2·1 to 3·7) at 1 year and 6·0 (4·9 to 7·2) at 3 years) and lymph node involvement ((y)pN2 versus (y)pN0) (2·5 (1·8 to 3·2) at year and 4·3 (3·2 to 5·3) at 3 years). No further increase in risk was observed at 5 years. In a subanalysis, tumour‐involved resection margin (R1 versus R0) was associated with M‐PM with a RD of 3·9 (1·6 to 6·2) at 1 year and 5·9 (2·6 to 9·3) at 3 years. CONCLUSION: The overall risk of M‐PM in patients with colorectal cancer is low, but is increased in advanced T and N status. Follow‐up of at least 3 years after colorectal cancer surgery may be necessary, given the potential curative treatment of early diagnosed M‐PM.