The results of our study revealed that higher surgeon volume was independently associated with the lower five-year mortality rate in CRC patients receiving definitive surgery after adjusting hospital volume and other potential confounders. When introducing different definitions of provider volume, the inverse association between surgeon volume and five-year mortality rate remained consistently significant, while hospital volume did not. Surgeon volume was more important than hospital volume in CRC surgeries. Thus, those CRC patients treated by high-volume surgeons at low-volume hospitals had a lower risk of mortality than those treated by low-volume surgeons at high-volume hospitals. In addition, our findings demonstrate that “cumulative volume” could predict the five-year mortality of the study cohort better than total and annual volume.
There is a large body of literature investigating the volume-outcome relationship in cancer treatment. There are lots of studies regarding the outcome-volume relationship that have been reported since 30 years ago [20]. However, the definition of provider volume used in these studies varied. A different definition of provider volume could affect analytic results of patient outcomes. The most common volume definitions are annual volume and cumulative volume [4, 8, 11, 21, 31]. In a volume-outcome study on cervical cancer patients after radiation therapy conducted by Wright et al., mean annualized hospital volume and previous year hospital volume were not associated with survival benefit, while in sensitivity analysis, current year hospital volume which was defined as the number of patients treated at a given hospital within the same calendar year, predicted survival outcome significantly [28]. Derogar et al. defined annual volume as the number of operations within the index year and cumulative volume as the chronological number of operations, and they found the combination of annual and cumulative surgeon volume was a predictor of long-term survival for esophageal cancer patients, whereas individual these factors were not [8]. Jeldres et al. reported that both annual and cumulative provider volumes were independent predictors of failure-free survival in patients with localized prostate cancer after definitive radiotherapy [12]. In the present study, we explored the effect of varying definitions of provider volume on the mortality rate of CRC patients and the results were consistent among different models. Moreover, we also reported AIC, BIC, and Harrell C statistics to examine the fitness of different volume definitions, which was not done by prior studies. When examining AIC, BIC, and Harrell C statistics in different volume definitions, cumulative volume, which was defined as the number of surgeries performed by the surgeon before the index surgery, is the best model-fitting. This number could present as an indicator of the cumulative experiences of surgery. In addition, this finding can also be used as a guide in subspecialty education programs that the cumulative procedure done by the surgeon should meet the minimal requirement to be certified as a specialist.
The positive volume-outcome relationship in CRC patients after surgery has been demonstrated in many of the literatures, with or without mutual adjustment for hospital and physician volume. Schrag et al. investigated both hospital and surgeon volume on the outcome of rectal cancer patients. The results of their work revealed that surgeon volume was a better predictor of long-term survival than hospital volume [26]. In the results from a Cochrane systematic review and meta-analysis, the effect of workload on patients’ prognosis after CRC surgery was stronger at the surgeon level than at hospital level, particularly in the outcome of 5-year overall survival and operative mortality [3]. Previous studies have explored the positive association of high cumulative and high annual volume surgeons on cancer survival rates for rectal and colorectal cancer patients [6, 31]. When exploring different volume definitions, the results of our study confirmed the robustness of the importance of physician volume over hospital volume on the long-term mortality rate in CRC patients after cancer surgery. These findings suggested that enhancing surgeon-specific experiences can improve patients’ outcomes. Thus, from a healthcare policy standpoint, minimal requirements for surgical procedures and specialization may be important for surgeon training in colorectal cancer surgery. In Taiwan, patients can seek second opinions with great accessibility under the coverage of National Health Insurance. As a result, patients tend to visit medical centers for cancer treatment. From the patients’ aspect, the information on surgeon volume can not only minimize the information gap but also help patients to choose an experienced surgeon in the local hospital near where they live, thus improving patient logistics of high-volume centers.
There were certain limitations of the current study. First, several potential confounders, including smoking, alcohol consumption, obesity, and family history, are not available in the data. Secondly, we lacked data on tumor markers and genetic features, which might also impact patients’ long-term survival. Third, potentially curative surgeries might be mixed with palliative operations. Fourth, surgical volume beyond the study period was not collected. Finally, a longer surgical experience may be an effect modifier for surgeon volume, which needs further studies.