Geographic variation in volumetric breast density between screening regions in the Netherlands

Objectives: Differences in breast density between populationsmay explain part of the variation in regional breast cancerscreening performance. This study aimed to determine whether regional differences in breast density distribution are present in the Dutch screening population. Methods: As part of the DENSE trial, mammographic density was measured using a fully-automated volumetric method. The regions in our study were based on the geographic coverage of 14 reading units representing a large part of the Netherlands. General linear models were used. Results: Four hundred eighty-five thousand and twenty-one screening participants with a median age of 60 years were included (2013-2014). The proportion of women with hetero-geneously or extremely dense breasts ranged from 32.5 % to 45.7 % between regions. Mean percent dense volume varied between 6.51 % (95 % confidence interval [CI]: 6.46-6.55)and 7.68 % (95 % CI: 7.66-7.71). Age differences could not explain the variation. Socio-economic status (SES) was positively associated with volumetric density in all analyses (lowSES: 6.95 % vs. high SES: 7.63 %;ptrend<0.0001),whereas a potential association between urbanisation and breast densityonly became apparent after SES adjustment. Conclusion: There appears to be geographic variation in mammographic density in the Netherlands, emphasizing the importance of including breast density as parameter in the evaluation of screening performance.