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Multiple Bilateral Circumscribed Masses (MBCM) at Screening Breast Ultrasound (US): Outcomes of New or Enlarging Masses at Follow-up.

Background: Screening US has limited specificity but is increasingly performed given widening state and federal breast density legislations. There is a need for evidence-based management guidelines. Objective: To assess outcomes of new or enlarging oval circumscribed parallel (OCP) masses in the setting of multiple bilateral circumscribed masses (MBCMs) at sequential rounds of US screening. Methods: In this HIPAA-compliant retrospective study of women found to have MBCMs on screening breast US without mammography abnormalities, longitudinal review was performed to identify development of any new or enlarging or changing masses. Outcomes were recorded using biopsy results or minimum 12 months of follow-up as reference standards. Lesion characteristics, BI-RADS classification, breast density, patient age, demographics, and risk factors were reviewed. Statistical analysis included multivariable logistic regression analysis. Results: There were 284 (2.4%) cases of MBCMs in a total of 48,488 bilateral screening US exams performed in 11,826 asymptomatic women during January 2014 and July 2019 that fit inclusion criteria. Of the 284 women (mean age, 46; range, 20-83), 150 (52.8%) subsequently developed 465 new, enlarging and/or changing masses, 125 of which underwent biopsy (26.9%). Of the 465 masses, 408 (87.7%) were OCP and similar to other MBCMs; and 57 (12.3%) were unique findings that were non-oval non-circumscribed masses. None of the new or enlarging OCP masses were malignant. In total, malignancy rate was 0% for women with MBCMs with follow-up (median, 40.8 months; range, 12-75 months) and 0% for those that underwent biopsy (n=284) (95%CI, 0%, 1.2%). Of women with concurrent MBCM and unique findings, 4 cancers were detected. Three were new irregular masses, and one previously oval mass changed in morphology to have new calcifications and irregular border. A younger age was related to the likelihood of having enlarging masses (p<0.001). Conclusions: In the setting of MBCMs, new or enlarging OCP masses are a common and benign event. Concurrent new irregular masses or previously oval masses that develop suspicious morphologic features should be carefully evaluated for malignancy. Clinical impact: Breast radiologists who encounter new or enlarging OCP masses with no suspicious morphologic change in the setting of MBCMs can safely defer biopsies.