In 1979, Luft et al. called for regionalisation of certain types of surgery after demonstrating a relationship between surgical volume and outcome (1). Although this was an intuitive observation implementation of the recommendations has been controversial. As recently as 2015 a consortia of hospitals in the US have pledged to meet minimum volume standards for certain surgery types including rectal cancer surgery (2).
Volume-outcome relationships have been studied with respect to surgeon volume, hospital volume and the interaction between each of these variables. In cancer surgery volume-outcome relationships may focus on measures of surgical safety such as morbidity and mortality, value and performance measures such as length of stay, and quality measures related to the oncologic outcome such as disease free and overall survival. An association between volume and outcomes has been reported for oesophagectomy (3,4), pancreatectomy (4) and hepatic resections (3).
Demonstrating surgeon or hospital volume-outcome relationships in colorectal cancer surgery has been unclear due to inconsistent results (5-8). Potential reasons for variation in the results could relate to the setting of the studies such as Medicare or Department of Veterans Affairs only, and geographical variation between states, countries and continents. In addition there might be variance depending on the site of surgery i.e. colon versus rectum, and type of surgery, open, laparoscopic or more recently robotic.
This review and meta-analysis aims to pool the current worldwide literature for the association between colorectal surgery outcomes with hospital volume and surgeon volume. This review will clarify several controversial issues including: (I) whether a hospital volume-outcome relationship exists in colorectal cancer resection; (II) whether a surgeon volume-outcome relationship exists; and (III) clarifying the association between high-hospital-volume-low-surgeon-volume versus low-hospital-volume-high-surgeon-volume.
The present systematic review and meta-analysis was performed according to the Meta-analysis of Observational Studies in Epidemiology (MOOSE) (9), PRISMA guidelines (10) and recommendations (9). Electronic searches were performed using Embase, Ovid Medline, PubMed, Cochrane Central Register of Controlled Trials (CCTR), Cochrane Database of Systematic Reviews (CDSR) and ACP Journal Club and Database of Abstracts of Review of Effectiveness (DARE) from their dates of inception to May 2016. To achieve maximum sensitivity of the search strategy and identify all studies, the following terms were combined as either keywords of MeSH terms: “volume”, “outcome”, “colorectal surgery”, “colon surgery”, “rectal surgery”, “morbidity”, “mortality”, “complications”, “hospital” and/or “surgeon”. The reference lists of all retrieved articles were reviewed for further identification of potentially relevant studies. All identified articles were systematically assessed using the inclusion and exclusion criteria. Retrieval of the full articles of all available studies were conducted, including direct contact with authors.
Eligible studies for the present systematic review and meta-analysis included those in which: (I) the subject of the study is the surgical treatment of colon, rectal or colorectal cancer; (II) hospital volume or surgeon volume is an independent variable tested; and (III) outcome parameters include post-operative mortality or survival. Studies that did not include mortality or complications as endpoints were excluded. When institutions published duplicate studies with accumulating numbers of patients or increased lengths of follow-up, only the most complete reports were included for quantitative assessment at each time interval. All publications were limited to those involving human subjects. There was no language restriction. Abstracts, unpublished studies, case reports, conference presentations, editorials, reviews and expert opinions were excluded.
Data extraction and appraisal
Two investigators independently conducted the study search [Y.R.H (BMed/MD, Honours), K.P. (MD, MPhil)]. The full articles of all relevant studies were extracted and evaluated for inclusion by both investigators. Discrepancies between the two reviewers were resolved by discussion and consensus with the two other authors [D.L.M (MD, PhD), W.L (MD)]. Study characteristics collected included year of publication, study period, primary country the study is based, population description, number of patients and number of hospitals. Outcomes extracted include 30-day mortality, in-hospital mortality, 5-year mortality, 5-year recurrence, local regional recurrence (LRR), postoperative complications, postoperative mortality, stoma rates, anastomic leak rates, and overall survival. Outcomes were extracted where possible as Hazard ratios (HR); 95% confidence interval (CI), and P values. Where possible, pooled outcomes were stratified by surgical procedure (rectal vs. colon vs. colorectal), and by the volume parameter (hospital procedure volume or surgeon volume).
Combined HRs for the association between hospital volume or surgeon volume with surgical outcomes were pooled under a random effects models. Heterogeneity analysis was performed using the Cochran Q test and I2 index. Computations were performed using Comprehensive Meta-Analysis v2 software (Biostat Inc, Englewood, New Jersey, USA).
Methodologic characteristics of included studies
The initial search identified 2,330 potentially relevant articles to which the exclusion and inclusion criteria were applied (Figure 1). After exclusion of duplicate or irrelevant references, 2,315 potentially relevant articles were retrieved. After detailed evaluation of these articles, 151 studies remained for detailed assessment. After applying inclusion and exclusion criteria, 47 studies (Table 1) were included for the present systematic review and meta-analysis, of which: 18 only examined surgeon and/or hospital volume-outcome outcomes for rectal surgery, 15 for colon surgery, 13 for colorectal surgery, and 1 that reported outcomes for colon and rectal surgery separately (Table 1). Assessment of the quality of included studies are demonstrated in Table S1. Overall, 1,122,303 patients underwent colorectal surgery (in 9,877 hospitals by 9,649 surgeons).
The characteristics of each included study is summarized in Table 1. Each study stratified “high” and “low” volume groups to different cut-offs. A majority of studies determined their cut-offs based on their study sample and divided the numbers evenly between groups (i.e., median if two groups, quartiles if 4 groups). For “low” hospital volume, cut-offs ranged from 5 or less operation per 5 years (37) to 530 or less operations annually (11). For the “high” hospital volumes group, cut-off ranged from 6 or more annually (51) to 2,623 or more operations annually (11). For “low” surgeon volume, cut-offs ranged from 1 operation per 5 years (37), to 108 or less annually (11). For “high” surgeon volume, cut-offs ranged from 6–26 per 5 years (37), to 561 or more annually (11).
Mortality: 30-day, in-hospital and intra-operative
The association between mortality with hospital volume and surgeon volume is summarised in Table 2 and Table 3 respectively. Thirty day mortality was significantly lower in hospitals with higher procedure volume compared to lower volumes (HR: 0.83; 95% CI: 0.78–0.87, P<0.001). Subgroup analysis demonstrated that this association remained significant for rectal surgery alone (HR: 0.81; 95% CI: 0.74–0.89) and colon surgery alone (HR: 0.67; 95% CI: 0.59–0.77) (Table 2).
Similarly, surgeons with a higher procedure volume had a significantly lower 30-day mortality for colorectal surgery than surgeons with a lower volume (HR: 0.84; 95% CI: 0.80–0.89) (Table 3). Subgroup analysis demonstrated this association remained significant for colon surgery and colorectal surgery, but not for rectal surgery (Table 3).
In-hospital mortality was significantly lower in high volume hospitals compared to low volume hospitals (HR: 0.93; 95% CI: 0.89–0.97, P<0.001) (Table 2). Similarly, high volume surgeons had significantly lower in-hospital mortality than low volume surgeons (HR: 0.98; 95% CI: 0.96–0.99, P<0.001) (Table 3).
Intra-operative mortality was reported by two studies which demonstrated significantly lower rates in high volume hospitals (HR: 0.82; 95% CI: 0.76–0.86, P<0.001) (Table 2), and high level surgeons (HR: 0.50; 95% CI: 0.40–0.62, P<0.001) (Table 3) compared to their respective low counterparts.
Detailed post-operative mortality for each surgeon and/or hospital volume group is listed in Table S2.
Five-year survival was significantly improved for high volume surgeons compared to low volume surgeons (HR: 0.79; 95% CI: 0.75–0.83, P<0.001) (Table 3). In comparison, 5-year survival was similar between low and high volume hospitals (HR: 0.99; 95% CI: 0.99–1.00). However, subgroup analysis demonstrated 5-year survival to be significantly better for high volume hospitals in the two studies which reported outcomes for colorectal surgery (HR: 0.91; 95% CI: 0.87–0.94, P<0.001) (Table 2). Detailed 5-year survival mortality for each surgeon and/or hospital volume group is listed in Table S2.
Local & 5-year recurrence
Local recurrence rates were reported in 5 studies (4 rectal and 1 colon surgery). Pooled results demonstrated local recurrence was significantly lower for high volume surgeons and high volume hospitals compared to low volume counterparts (HR: 0.71; 95% CI: 0.62–0.82, P<0.001 & HR: 0.55; 95% CI: 0.50–0.68, P<0.001 respectively) (Tables 2,3). Subgroup analysis demonstrated both associations were only significant for rectal surgery, not colon surgery. Five-year recurrence was similar for low and high volume hospitals (Table 2). No studies reported 5-year recurrence rates with surgeon procedure volume.
Compared to low-volume hospitals, high volume hospitals had significantly lower post-operative complications (HR: 0.89; 95% CI: 0.81–0.98, P<0.05) (Table 2). There was no significant difference in post-operative complications between high and low surgeon procedure volumes (HR: 0.85; 95% CI: 0.70–1.08) (Table 3).
High procedure volume surgeons had significantly lower rates of anastomotic leak (HR: 0.59; 95% CI: 0.37–0.94, P<0.01) (Table 3). There were no differences in the incidence of anastomotic leaks between low and high volume hospitals (Table 2). However, subgroup analysis revealed that for rectal only resections, high volume hospitals had significantly lower rates of anastomotic leak compared to low volume hospitals (HR: 0.75; 95% CI: 0.58–0.97, P<0.05) (Table 2).
Trend between hospital/surgeon volume with 30-day mortality and 5-year survival
Rather than just “low” and “high” groups, a number of studies reported outcomes in three or more continuous volume groups to demonstrate a stepwise change in outcomes (Table S2). A total of 15 studies reported 30-day mortality rates for three or more consecutive increases in hospital volume. 30-day mortality was found to decrease with each stepwise increase in hospital volume in 87% (13/15) of studies (Figure 2). Furthermore, 5-year survival was found to increase with each stepwise increase in hospital volume in 91% (10/11) of studies (Table S2).
On the other hand, a total of 12 studies reported 30-mortality rates for three or more consecutive increases in surgeon volume. 30-day mortality was found to decrease with each stepwise increase in surgeon volume in 83% (10/12) of studies (Table S2, Figure 3). Furthermore, 5-year survival was found to increase with each stepwise increase in surgeon volume in 100% (6/6) of studies (Table S2).
Interaction between surgeon volume and hospital volume
Four studies (25,38,41,46) examined the interaction between surgeon and hospital volume on outcomes. High-hospital-high-surgeon-volume had the best 5-year survival [78% in Renzulli et al. (25)] and lowest mortality rates [(3.2% in Callahan et al. (38)]. Low-hospital-high-surgeon-volume had better 5-year survival than high-surgeon-low-surgeon-volume [76% vs. 68% in Renzulli et al. (25)], and lower mortality rates in Callahan and colleagues’ study (14) (4.2% vs. 5.0%). However, one study (41) reported low-hospital-high-surgeon-volume had a higher mortality rate than high-surgeon-low-surgeon-volume (Table S3). Low-hospital-low-surgeon-volume had the highest mortality rate and lowest 5-year survival outcome in all four studies (Table S3).
Liu et al. (11)  identified hospital stay was significantly shorter with high volume surgeons whereby patients were more likely to be discharged within 14 days post-operatively (HR: 0.67; 95% CI: 0.46–0.99, P<0.05) (Table 3). Hospital volume was not associated with shorter hospital stay (Table 2).
Yasunaga et al. (17)  reported operating time was significantly reduced with high volume surgeons compared to low volume surgeons for colon surgery (HR: 0.17; 95% CI: 0.10–0.29, P<0.001) (Table 3). Hospital volume was not found to be associated with operating time (Table 2).
Liu et al. (11)  defined hospital expenses as the total expenses during hospitalization and related expenditures for a definitive surgery. Interestingly, Liu and colleagues reported hospital expenses was higher with high volume hospitals (OR: 1.73; 95% CI: 1.33–2.25, P<0.001) (Table 2), but lower with high volume surgeons (OR: 0.29; 95% CI: 0.18–0.44, P<0.001) (Table 3).
Adequate lymph nodes resected
Richardson et al. (13)  defined adequate lymph node resection as 12 lymph nodes in rectal surgery. The study reported high volume surgeons were more likely to have adequate lymph node resected compared to low volume surgeons (50% vs. 23% respectively, HR: 2.66; 95% CI: 2.36–5.70, P=0.01) (Table 3). No study examined the association between lymph node resection and hospital volume.
This review and meta-analysis demonstrates that there is a volume-outcome relationship that favours high volume facilities and high volume surgeons. Higher hospital and surgeon volume resulted in reduced overall, in-hospital and intra-operative mortality. Post-operative complication rates depended on hospital not surgeon volume except with respect to anastomotic leak. Notably, high volume surgeons are associated with improved oncologic outcomes including greater lymph node retrieval, reduced recurrence rate and 5 year survival. Higher volume surgeons also improve value through reduced operative time, length of stay and cost. The best outcomes occur in high volume hospitals with high volume surgeons.
It is interesting that this meta-analysis demonstrates the rates of mortality are not the lowest in the studies with the highest hospital or surgeon annual case load. For example, studies where the highest volume had more than 100 operations annually than the lowest group had smaller reduction in mortality between groups than studies where the highest group had more than 20 operations annually than the lowest group. This makes it difficult to identify a clear threshold of effect, or a mathematical relationship between increasing volume and improvement in any of the outcome measures. A potential reason for this may be the highest volume hospitals have more surgeons, and therefore, each individual surgeon volume is low, whilst the smaller hospitals only have a few surgeons, and each individual surgeon volume is high. One study (41) reported low-hospital-high-surgeon-volume had a higher mortality rate than high-surgeon-low-surgeon-volume. However, three included studies demonstrated that low-hospital-high-surgeon-volume had better 5-year survival and lower risk of mortality than high-surgeon-low-surgeon-volume (25,38,46) (Table S3). This suggests that high surgeon volume is more critical than high hospital volume to optimise outcomes.
The mortality and 5-year survival varied between studies due to the different patient populations between and within the studies. The gold-standard for comparing surgical outcomes is to adjust for risk related to underlying patient characteristics that might influence the outcome (53). In the identified publications there was major variation in controlling for patient, cancer, hospital and surgeon characteristics. In addition, different risk measures such as Charlson score or ASA were used in different studies. Maruthappu et al. (53) has demonstrated overall poor methodological qualities for studies assessing individual surgical performance. Future studies should seek to control for patient characteristics and utilise risk stratification.
Numerous other factors may contribute to surgical outcomes. Some of these are directly related to volume, e.g., learning curve, and potentially explain the lack of continuous relationship between volume and outcome for surgeons or centres. It is worth noting that the annual volumes reported in this manuscript are often lower than those in studies of learning curve for colorectal surgery (54). This perhaps suggests that once initial learning curve is overcome then further improvement in outcomes is likely to be in small increments.
There has been a trend towards providing feedback to surgeons and physicians about their performance. It has been thought that feedback in its own right will lead to reflection and efforts to improve outcomes. This has been controversial particularly in the context where league tables are reported publically and linked to reimbursement. Recent studies comparing outcomes of hospitals participating in the American College of Surgeons’ National Surgical Quality Improvement Program (NSQIP) to non-NSQIP hospitals found no difference in rate of improvement in outcomes or in morbidity/mortality rates for a variety of general surgical procedures (55). Notably, however, there was across the board improvement overtime in both groups. Some suggest it may reflect a generally increased interest in quality improvement (55). However, as reasons for this are speculative, future research in this area is necessary to determine the most appropriate methods improve patient outcomes.
Volume outcome relationships have been challenged in other settings. Kurlansky et al. (56) demonstrate that low volume centres and low volume surgeons can have good outcomes if they are compliant with evidence-based quality standards (56). It is recognised that quality improvement requires multi-factorial interventions such as appropriate feedback and education (57). It may be that high volume for individual surgeons or high volume hospitals may actually be a surrogate for quality interventions such as protocolisation of care pathways, however future research to confirm this is warranted.
This study has limitations that need to be considered when interpreting the results. Firstly, there is little overlap between the cut-off points that define low and high volume groups between studies. As each health district and country holds different health service to population ratios, what is considered “high volume” is substantially different to another. This makes it difficult to demonstrate a universal cut-off level for minimum amount of operations annually to optimise outcomes. However, with most studies, the increase in surgeon and hospital volume is associated with a trend for reduced adverse outcomes, irrespective of absolute volume load. Secondly, some of the studies from USA used databases such as Medicare, SEER-Medicare, NCDB and California Cancer Registry which have overlapping patients. This may result in some patients being included multiple times in this meta-analysis. The studies from other countries such as Taiwan and Germany do not have this limitation as they are from independent hospital databases. Finally, heterogeneity exists between the studies in terms of the types of colorectal surgeries performed, distribution of cancer stages among hospitals and surgeons of various volumes. It is possible that those surgeons and hospitals with the highest volume perform more complex, high-risk operations. This would under-estimate the benefit of a high volume load over small volume load. Nevertheless, even with increasing volume loads, and the potential increase in more complex operations performed, a majority of the studies demonstrate a decrease on adverse outcomes with increasing volumes.
In conclusion, this study confirms that surgeon and hospital volume impacts colorectal surgical outcomes but fails to demonstrate a linear relation between volume and outcome. This suggests that whilst centralisation of such services may represent good policy, volume alone will not guarantee good or improved outcomes. Low and high volume surgeons and centres should seek to address quality through multiple methods, not just by increasing volume. Further studies on volume should better describe the quality improvement context of the operator such as participation in improvement programs, performance feedback and compliance with guidelines.
Conflicts of Interest: The authors have no conflicts of interest to declare.
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