Mammography screening has limited performance in young women or women with dense breasts although it is the only proven method for breast cancer screening that reduces the mortality. The investigators propose a multicenter trial of breast cancer screening to assess the effectiveness of supplemental ultrasonography for Korean women aged 40-59 years.

The investigators will screen same consecutive participants using both methods; digital mammography plus supplemental ultrasonography (intervention arm) and digital mammography only (control arm) for 5 years. The investigators will collect follow-up information by assessment of screening records, questionnaire, and official cancer registry. The primary outcome will be sensitivity, specificity, recall rate, cancer detection rate and stage distribution at the end of first round of screening. The secondary outcome will be cost-effectiveness and cost-utility of digital mammography with ultrasonography versus digital mammography only for breast cancer screening. It is hoped that the results of this trial will provide guidance of effective breast cancer screening strategy to women aged 40-59, especially with dense breasts.

Inclusion Criteria:

• Women aged 40-59 years
• No history of any cancer
• No breast mammography (and ultrasonography, in case of intervention arm) after the introduction of digital mammography

Exclusion Criteria:

• Interstitial injection
• Women with history of any cancer

Women aged 40-59 years at target population in National Cancer Screening Program