Objective  To investigate the imaging features of primary adenoid cystic carcinoma (ACC) of the breast.

  Methods  In total, 31 patients were confirmed to have primary ACC in the breast based on histopathological findings from January 2012 to September 2018 in Tianjin Medical University Cancer Institute and Hospital. Mammography and ultrasonography findings of these patients were retrospectively analyzed according to the breast imaging-reporting and data system (BI-RADS) recommended by the American College of Radiology (ACR) and compared with pathological findings.

  Results  Mammography findings in 31 cases of ACC of the breast were as follows:25 cases with masses, including 16 high-density masses (14 were categorized as BI-RADS 4C or 5, one as 4A, and one as 4B); 7 equal-density masses (5 were categorized as BI-RADS 4B, one as 4A, and one as 3); and 2 mixed-density masses (one was categorized as BI-RADS 4A and one as 2). The other 6 patients presented with focal asymmetric density. No calcification was found in all cases. Ultrasonography findings included hypoechoic mass, mixed-echo mass, and non-mass-like hypoechoic or heterogeneous echoic areas. A total of 22 patients presented with hypoechoic masses; of these, 14 showed typical ultrasonographic findings of breast cancer and their tumors were categorized as BI-RADS 4C or 5. Three patients presented with mixed-echo masses, of which two showed a mixed-echo mass containing hyper-echoic areas and one showed a complex cystic and solid echo mass; all tumors were categorized as BI-RADS 4B. The remaining 6 patients presented with non-mass-like hypoechoic or heterogeneous echoic areas; all were without a tendency to distribute along the direction of the breast ducts.

  Conclusions  The mammographic and ultrasonographic appearances of primary ACC of the breast are similar to those of general carcinoma, with no specificity. Some specific findings, such as irregular high-density masses containing low- or fatty-density areas with indistinct margins on mammography and mixed-echo masses containing hyper-echo or non-mass-like heterogeneous echoic areas not distributed along the breast duct on ultrasonography, have certain significance. The final diagnosis depends on histopathology.