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.
(© 2020 The Authors. BJS Open published by John Wiley & Sons Ltd on behalf of BJS Society Ltd.)