Abstract: Overall five year survival was significantly improved for patients with colorectal cancer treated in high-volume hospitals (HR=0.90, 95% CI 0.85 to 0.96), by high-volume surgeons (HR=0.88, 95% CI 0.83 to 0.93) and colorectal specialists (HR=0.81, 95% CI 0.71 to 0.94). Operative mortality was significantly better for high-volume surgeons (OR=0.77, 95% CI 0.66 to 0.91) and specialists (OR=0.74, 95% CI 0.60 to 0.91), but there was no significant association with higher hospital caseload (OR=0.93, 95% CI 0.84 to 1.04) when only case-mix adjusted studies were included. There were differences in the effects of caseload depending on the level of case-mix adjustment and also whether the studies originated in the US or in other countries. For rectal cancer, there was a significant association between high-volume hospitals and improved 5-year survival (HR=0.85, 95% CI 0.77 to 0.93), but not with operative mortality (OR=0.97, 95% CI 0.70 to 1.33); surgeon caseload had no significant association with either 5-year survival (HR=0.99, 95% CI 0.86 to 1.14) or operative mortality (OR=0.86, 95% CI 0.62 to 1.19) when case-mix adjusted studies were reviewed. Higher hospital volume was associated with significantly lower rates of permanent stomas (OR=0.64, 95% CI 0.45 to 0.90) and APER (OR=0.55, 95% CI 0.42 to 0.72). High-volume surgeons and specialists also achieved lower rates of permanent stoma formation (0.75, 95% CI 0.64 to 0.88) and (0.70, 95% CI 0.53 to 0.94, respectively). The results confirm clearly the presence of a volume-outcome relationship in colorectal cancer surgery, based on hospital and surgeon caseload, and specialisation. The volume-outcome relationship appears somewhat stronger for the individual surgeon than for the hospital; particularly for overall 5-year survival and operative mortality, there were differences between US and non-US data, suggesting provider variability at hospital level between different countries, making it imperative that every country or healthcare system must establish audit systems to guide changes in the service provision based on local data, and facilitate centralisation of services as required. Overall quality of the evidence was low as all included studies were observational by design. In addition there were discrepancies in the definitions of caseload and colorectal specialist. However ethical challenges associated with the conception of randomised controlled trials addressing the volume outcome relationship makes this the best available evidence.

Plain-language summary: After thorough search of the available literature, we found fifty-four observational studies (fifty one meta-analysed) including 943,728 patients that addressed either the volume-outcome relationship in the context of modern colorectal cancer treatment, or the effects of surgeon specialization. The results confirm the presence of a volume-outcome relationship in colorectal cancer surgery, based on hospital and surgeon caseload, and benefits for specialization. For death within five years of treatment, hospital volume appeared to be more beneficial in rectal cancer surgery than for colon cancer. However, international differences in the data suggest provider variability at the hospital level between the different countries, making it imperative that every country or healthcare system must establish audit systems to guide changes in the service provision based on local data, and facilitate centralization of services as required. Overall quality of the evidence was low as all included studies were observational by design. In addition there were discrepancies in the definitions of caseload and colorectal specialist. However ethical challenges associated with the conception of randomised controlled trials addressing the volume outcome relationship makes this the best available evidence.