An analysis of factors associated with interval as opposed to screen-detected breast cancers, including hormone therapy and mammographic density.

A comparison of 270 interval breast cancers in South Australian women aged 50 years and over with 628 age-matched screen-detected cases indicated that the former had more advanced stages (P<0.001), higher grades (P<0.001), and more frequently a history of past breast problems (P<0.027). After adjusting for these factors, and presence of a self-reported breast lump at the last screen, using conditional logistic regression, hormone replacement therapy (HRT) exposure in the 6 months prior to this screen had a raised relative odds (95% CL) of an interval cancer of 1.48 (1.02, 2.14). For 479 women where breast density was measured, high density showed an elevated relative odds of an interval cancer of 2.62 (1.71, 4.02). The relative odds of a high density was raised to 2.02 (1.33, 3.06) when the HRT history was positive. Screeners should be aware when there is a history of HRT or past breast problems, or a high breast density, that there is an increased probability of a subsequent interval cancer.