Borderline ovarian tumors in Denmark 1997-2018: Time trends in incidence by histology, age and educational level

Introduction After some decades with an increasing incidence of borderline ovarian tumors, more recent studies have observed that the incidence rate seems to be leveling off or declining. In this study, we describe the incidence of borderline ovarian tumors in Denmark 1997-2018 by histology, age at diagnosis and educational level. Material and methods All borderline ovarian tumors registered in the Danish Pathology Registry during 1997-2018 were identified and individual-level educational information was retrieved from nationwide registers. Age-standardized incidence rates were estimated according to histology, age at diagnosis and educational level. To investigate incidence trends over time, the average annual percentage change and corresponding 95% confidence intervals (CIs) were estimated using Poisson regression. Results We identified 3927 women with borderline ovarian tumors during the study period, of which 1997 (50.9%) were serous and 1743 (44.4%) were mucinous. The age-standardized incidence rate of serous borderline ovarian tumors did not change significantly over calendar time (average annual percentage change = -0.13, 95% confidence interval [CI] -1.13 to 0.88). For mucinous tumors, the age-standardized incidence rate was also relatively stable during the first half of the study period, followed by a decrease from 2.56 to 1.25 per 100 000 person-years between 2007-2011 and 2017-2018. Over the entire study period, the incidence rate of mucinous borderline tumors declined on average by 2.91% (95% CI -4.24 to -1.51) per year. The incidence of both types of borderline ovarian tumors seemed to be highest among women with a low educational level. Over calendar time, the incidence of mucinous tumors decreased in all educational groups, whereas the incidence of serous tumors decreased exclusively in women with a high educational level. Time trends did not differ markedly by age at diagnosis. Conclusions In Denmark, the incidence of serous borderline ovarian tumors was stable during 1997-2018, whereas the incidence of mucinous borderline ovarian tumors decreased. The incidence rates of both types of borderline ovarian tumors tended to be highest among women with a low educational level throughout the study period.