Overall upstaging rate
From the 23 studies, a total of 5194 EOC patients between 1975 and 2020 with clinically stage I or II were identified, of which 931 were upstaged based on surgical-pathological findings. The upstaging rate within the individual studies varied from 4.3% to 38.5%. The pooled meta-analytical estimate was 18.7% (95%CI: 14.1-23.4%) (see Appendix Figure S3). A metaregression analysis on the year of inclusion did not provide a significant association (p=0.188) with the overall upstaging rate (-0.4% per year; 95%CI: -0.9-;0.2%) (data not shown). Eleven studies investigated 3627 patients with clinically stage I EOC only, which resulted in an upstaging rate of 23.0% (95%CI: 15.5-30.5%). The other twelve studies included a mix of patients with clinically FIGO stage I and II EOC in which 14.5% (95%CI: 9.6-19.4%) out of 1567 patients were upstaged (see Appendix Figure S4). This difference between subgroups is not statistically significant (p=0.771).