Objective: To determine the rate of malignancy of probably benign mass and masses (BI-RADS3) detected by screening mammography with adjunctive breast ultrasound at Phramongkutklao Hospital.

Materials and Methods: Probably benign breast mass and masses detected by screening mammography with adjunctive breast ultrasound at Phramongkutklao Hospital between January 1 and August 31, 2012 and their follow-up were retrospectively reviewed with the hospital board approval. The study was conducted until either the stability of the mass was found at follow-up of at least two years or tissue diagnosis was needed. The rate of malignancy was then calculated.

Results: The study population was 241 women with 675 probably benign masses. After at least two years of follow-up, (usually at 6, 12 and 24 months), 667 masses showed no interval enlargement or any suspicious sign. Eight masses turned out to be suspicious lesions (BI-RADS4 or 5), all of which received core biopsies. One malignancy was found, at 17-month follow-up. It was early cancer, T1N1M0. The rate of malignancy was 0.41%.

Conclusion: The rate of malignancy of probably benign mass and masses (BI-RADS3) detected by screening mammography with adjunctive breast ultrasound at Phramongkutklao Hospital was low (0.41%). The malignancy was found at 17-month follow-up and was in early stage. Therefore, a recommendation of a 12-month follow-up may be more appropriate than a 6-month interval.

Keywords: Probably benign mass and masses (BI-RADS3), Screening mammography with adjunctive breast ultrasound, Rate of malignancy