Biennial screening mammography: How many women ask for more? Estimate of the interval mammogram rate in an organised population-based screening programme

Objectives: To estimate the interval mammogram rate, i.e. the undertaking of an additional mammography between scheduled screening rounds, and identify factors influencing this phenomenon. Methods: Data from our screening programme for the year 2014, excluding prevalent rounds, were analysed. Information about the number of women who underwent interval mammograms was obtained reviewing the questionnaires and searching the department database. Data on age, breast density, family history of breast cancer, number of screening rounds, previous recalls, general practitioner, and city of residence (used as a proxy of local socio-economic differences) were evaluated using chi-square test. Results: Of 2780 screened women (incident rounds), 2566 had complete data (92%). The interval mammogram rate was 384/2566 (15%, 95% confidence interval 14–17%). Women classified with American College of Radiology c or d breast density categories showed a higher interval mammography probability than those with a and b density (p < 0.001); women in their second round showed a higher probability of interval mammogram compared to women in their fifth, sixth, or seventh round (p ≤ 0.004). No significant differences were found between women with and without an interval mammogram when considering previous recalls for a negative work-up (p = 0.241), positive breast cancer family history (p = 0.538), and city of residence (p = 0.177). Conclusions: The interval mammogram rate was relatively low (15%). Higher breast density and first of years of adherence to the programme were associated with higher interval mammogram rate.