Setting the record straight—Correcting uterine cancer incidence and mortality in the Nordic countries by reallocation of unspecified cases

Abstract

Introduction The incidence of and mortality from cancers of the cervix uteri and corpus uteri are underestimated if the presence of uterine cancers, where the exact topography (site of origin) is not specified, is omitted. In this paper we present the corrected figures on mortality from and incidence of cervix and corpus uteri cancers in the Nordic countries by reallocating unspecified uterine cancer deaths and cases to originate either from the corpus uteri or cervix uteri. To further validate the accuracy of reallocation, we also analyzed how well the reallocation captures the changes occurring as the result of a transition in cause of death coding in Norway that took place in 2005. Material and Methods This study uses data available in the NORDCAN database, which contains aggregated cancer data from all the Nordic countries for the years 1960–2016. The unspecified uterine cancer cases and deaths were reallocated to either cervix uteri or corpus uteri based on the estimated probability that follows the distribution of cases and deaths with verified topography. The estimated proportions of cases and deaths for both cancers were calculated for each combination of age group, year, and country as a proportion of cases (and deaths, respectively) with known topography. Annual age-standardized rates were calculated by direct age-adjustment. Results The proportions of unspecified uterine cancers were higher in the mortality data than in incidence data, with mean values for 1960–2016 ranging between 5.1% and 26.6% and between 0.2% and 6.8% by country, respectively. In the Nordic countries combined, the reallocation increased the number of cases by 4% and deaths by approximately 20% for both cancers. Finland was the only Nordic country where the mortality rate did not increase substantially after reallocation. Conclusions The reallocation procedure had a significant impact on mortality from cancers of the cervix and corpus uteri for countries where the proportion of cancer deaths coded as uterus, not otherwise specified, is substantial. More effort to validate cause of death data with incidence data from cancer registries is warranted to avoid erroneous conclusions of temporal trends based on uncorrected cancer burden.