Association between mammographic breast composition and breast cancer risk among Japanese women: a retrospective cohort study.

Background: Mammographic breast composition is associated with breast cancer risk. However, evidence in a Japanese cohort investigating this association is scarce. Thus, we aimed to compare breast cancer risk between women with and without dense breasts.

Methods: All Japanese women who underwent breast cancer screening at a tertiary care academic hospital-affiliated preventive center at least twice with known baseline mammographic breast composition were included in this study. A single-center retrospective cohort study was conducted among 24,863 women who had 125,566 screening opportunities between April 1, 2005, and March 31, 2015. All women were categorized into two groups based on their baseline breast composition: women with dense breasts (13,815) and women with non-dense breasts (11,048). We compared the demographic characteristics between the two groups. After calculating person-years, Cox proportional hazards analyses were performed to estimate the hazard ratio (HR) of developing breast cancer according to breast composition status.

Results: During the study period, 358 breast cancer cases were identified. The dense and non-dense groups differed significantly by age, body mass index, family history of breast cancer, physical activity, history of smoking and alcohol consumption, number of pregnancies, and number of deliveries. After adjusting for these factors, Cox proportional hazards analyses showed that women with dense breasts had a significantly higher HR for developing breast cancer than women without dense breasts. The association was even stronger in younger women (≤ 50 years old), but it did not achieve statistical significance in older women.

Conclusion: Dense breasts at baseline are a risk factor for developing breast cancer in Japanese women. However, this association was only observed in women aged 50 years or younger at the time of entry into the screening program.