FAST MRI: DYAMOND trial protocol (can an abbreviated MRI scan detect breast cancers missed by mammography for screening clients with average mammographic density attending their first screening mammogram?)-a diagnostic yield study within the NHS population-risk breast screening programme.

Introduction: First post-contrAst SubtracTed (FAST) MRI, an abbreviated breast MRI scan, has high sensitivity for sub-centimetre aggressive breast cancer and short acquisition and interpretation times. These attributes promise effective supplemental screening. Until now, FAST MRI research has focused on women above population-risk of breast cancer (high mammographic density or personal history). DYAMOND aims to define the population within the population-risk NHS Breast Screening Programme (NHSBSP) likely to benefit from FAST MRI. The study population is the 40% of screening clients aged 50-52 who have average mammographic density (BI-RADS (Breast Imaging Reporting and Data System) B) on their first screening mammogram. DYAMOND will answer whether sufficient numbers of breast cancers, missed by mammography, can be detected by FAST MRI to justify the inclusion of this group in a future randomised controlled trial.

Methods and analysis: Prospective, multicentre, diagnostic yield, single-arm study with an embedded qualitative sub-study: all recruited participants undergo a FAST MRI. An internal pilot will assess the willingness of sites and screening clients to participate in the study. Screening clients aged 50-52, with a clear first NHSBSP mammogram and BI-RADS B mammographic density (by automated measurement) will be invited to participate (recruitment target: 1000). The primary outcome is the number of additional cancers detected by FAST MRI (missed by screening mammography). A Fleming's two-stage design will be used as this allows for early stopping after stage 1, to save participants, funding costs and time continuing to the end of the study if the question can be answered earlier.

Ethics and dissemination: The NHSBSP Research and Innovation Development Advisory Committee and the Yorkshire and Humber-Sheffield Research Ethics Committee (23/YH/0268, study ID (IRAS): 330059) approved this research protocol. Participation involves a two-stage informed consent process, enabling screening for eligibility through automated mammographic density measurement. Patients with breast cancer helped shape the study design and co-produced participant-facing documents. They will disseminate the results to the public in a clear and meaningful way. Results will be published with open access in international peer-reviewed scientific journals.

Trial registration number: ISRCTN74193022.