In this study, the institutional case-volume of ovarian cancer surgery was identified as an independent risk factor for in-hospital or 1-year mortality. No definite correlation was found between case volume and 3-year or 5-year mortality.
The standard treatment for ovarian cancer has been surgery with an aim for complete tumor removal followed by adjuvant chemotherapy. The use of neoadjuvant chemotherapy or radiotherapy is limited to specific circumstances and require more evidence [15-16]. Ovarian cancer can be considered as a high-risk surgery as more than 70% of the patients are diagnosed in the advanced stage with cancer involvement of the peritoneal cavity and adjacent organs [6, 17]. Extensive resections including salpingo-oophorectomy, hysterectomy, omentectomy, and occasionally resection of the bowel, bladder, liver, spleen, diaphragm or other organs are required to achieve maximal cytoreduction which is associated with long-term prognosis [18, 19]. A relatively small prospective observational study of 275 patients in Finland demonstrated that higher hospital operative volume (>20 cases/year) was associated with more optimal cytoreduction and a significant prognostic factor for 5-year survival . Recently, complete resection of all macroscopic tumor was emphasized as the cornerstone for ovarian cancer treatment as residual tumor of even less than 1 cm in diameter was associated with worse prognosis compared to complete resection [5, 20].
Previous studies have reported conflicting results regarding the association between the surgical case volume of ovarian cancer and postoperative outcomes [11-14]. An analysis of 100,725 patients using the National Cancer Database of the US, both 2- and 5-year survival were lower in low-volume centers (1-2 cases/year) compared to high-volume centers (≥20 cases/year) after categorizing the hospitals into quintiles according to annual case volume (64.4% vs. 77.4% for 2-year survival, P <0.001; 39.3% vs. 51.0% for 5-year survival, P <0.001) . Another report that investigated advanced-stage epithelial ovarian cancer showed that hospital volume ≥21 cases/year was a strong predictor of 5-year overall survival after analyzing 45,929 patients from the National Cancer Database of the US . To the contrary, another study that evaluated 2,952 patients aged 65 years or older did not show any association between hospital surgical volume and 60-day mortality or overall survival when hospitals were grouped into tertiles based on hospital surgical volume (cutoff value of 13 and 29 cases/year) between 1992 to 1999 . While prior studies defined high-volume center as an annual volume 20 or more cases per year, the determined cutoff value in our study was 30 and 100 cases per year for medium- and high-volume center respectively after visual inspection of the distribution of the annual case volume and number of patients. It seems likely that there may be another cutoff past 20-29 cases per year, beyond which patient outcome may improve, yet again.
Ovarian cancer surgery is often accompanied by significant postoperative morbidity. According to a previous analysis of 28,651 women who underwent ovarian cancer surgery, the complication rate ranged from 17.1% in patients under 50 years to 31.5% of in patients over 80 years of age . Another study reported that severe postoperative complications within 90 days after primary debulking surgery for advanced epithelial ovarian cancer showed that events requiring invasive interventions or re-operation, organ systems failure, or postoperative death occurred in 22.3% (138/620) of patients . In another study that classified hospitals into tertiles based on case volume, failure to rescue rates (defined as the mortality following a major complication), were higher in patients treated at low-volume hospitals compared to patients treated in high-volume hospitals (8.0% vs. 4.9%, P <0.001) . In this regard, accumulated experience in coping with postoperative complications may play an important role in case-volume effect.
Unlike previous reports, the surgical case-volume effect was not demonstrated at 3 or 5 years after surgery in the present study [12, 21]. Potential mechanisms behind the dilution of case volume effect over time may include the relatively high level of access to care. As a small country with complex transportation infrastructure, large hospitals which are high volume centers can be accessed from anywhere in the country within 3-4 hours. It is also relatively easy to seek care in high volume centers as the requirements are easy to meet. Considering that the stage of the ovarian cancer at the time of surgery was not available in the data, and therefore not adjusted for, concentration of patients in advanced stages in high volume centers may have contributed to the dilution of the case volume effect.
Case-volume effect on patient outcome has been demonstrated mainly in high-risk procedures such as major cancer resections, cardiovascular procedures, and heart transplantation [7-10] while relatively simple surgical procedures such as laparoscopic cholecystectomy showed no significant case-volume effect [25, 26]. Experienced surgical teams, nurses, and anesthesiologists are more likely to be prepared to optimally manage anticipated and unanticipated complications that may occur during high-risk surgical procedures. With regards to ovarian cancer surgery, collaboration with other specialists such as general surgeons, urologic surgeons, or thoracic surgeons are often required for optimal treatment. In this respect, comprehensive perioperative care by multidisciplinary team consisting of sufficient and skilled medical personnel as well as accumulated experience of individual surgeons may explain, to some extent, the case-volume effect in high-volume centers. The suggested amount of experience that may lead to the best patient outcome in this study was 100 cases of ovarian cancer surgery per year.
Several limitations should be considered when interpreting the result of this study. First, this study was a retrospective analysis of an administrative database. Therefore, the results may be subject to biases and limitations of retrospective studies and database not designed/collected for research. However, the database used in our study contains data from all 23,487 patients who received surgery for ovarian cancer in Korea during the study period, which may compensate for the shortcomings stated above and provide a relatively strong explanatory power. Second, clinical information such as staging, histologic diagnosis, or laboratory findings were not available. Although these clinical variables are likely to influence patient treatment and subsequent outcomes, individual clinical data could not be obtained due to the nature of the administrative database. Potential confounders that could be identified such as operation type, adjuvant therapy, and comorbidities were adjusted for. Third, the determined cutoff values of volume criteria were not previously validated. However, cutoff criteria for case volume may be bound to differ depending on the healthcare system, geographical circumstances, and the type of surgical procedure. Therefore, cutoff values for determining case volume effect may not need external validation as it would not be feasible to apply cutoff values determined from studies performed in different settings and time.