For the investigation of the relationship between VoS and quality of treatment outcome in complex oesophageal interventions (research question 1), a total of 37 observational studies were included, of which 30 contained usable data. Only 1 study had a high informative value of results.

For hospital VoS, a correlation between VoS and quality of treatment outcome was found for several operationalizations regarding the outcome of mortality. For the outcome of treatment-related complications (anastomotic insufficiency), likewise, a correlation was found between VoS and quality of treatment outcome on the basis of a study with high informative value of results. A correlation was also found with regard to the outcome of failure to rescue. For the additionally defined outcome of rehospitalization, a study with low informative value of results revealed a correlation between VoS and quality of treatment outcome to the disadvantage of high-VoS hospitals.

Regarding physician VoS, for the outcome of mortality, a correlation between VoS and quality of treatment outcome was found only for the operationalization of inpatient mortality. For the outcome of treatment-related complications (anastomotic insufficiency), a study of high informative value of results likewise showed a correlation between VoS and quality of treatment outcome.

No studies of meaningful interpretive value were found to investigate the extent to which the quality of treatment outcome is impacted by specific minimum numbers of cases introduced in the healthcare system for complex oesophageal interventions (research question 2).