These findings come from a study set up to examine the association between cancers found between screenings (interval cancers), breast density and the reported use of HRT. Although it is already known that the use of HRT can make breasts denser, this study found that more HRT users than expected had dense breasts, and that interval cancers were detected more frequently than expected amongst women who had used HRT.
Breast density was divided into three categories: lucent (where there was less than 30% glandular tissue - the least dense), intermediate (30-70% glandular tissue), and dense (more than 70% glandular tissue). Six radiologists were asked to scrutinize a mixed set of mammograms which consisted of subsequent interval cancers, screening normal, and screening detected cancers. Information about whether the women had ever used HRT was collected by questionnaire.
The radiologists made some significant findings. Women who had ever used HRT were more likely to have dense breasts and to have cancers detected between screenings, while women who had never used HRT were more likely to have lucent breasts and either to have normal screenings or for tumours to be detected by mammography.
Mrs Wang reported: Our finding that interval cancers are associated with dense breasts and the use of HRT underlines how the efficacy of screening can be affected by high breast density and HRT.