Background  Adenoid cystic carcinoma of the salivary gland is a common malignant tumor of the salivary gland with frequent local recurrences and distant metastases. It is often misdiagnosed as pleomorphic adenoma due to its resemblance to benign pleomorphic adenoma in shape and characteristics.

  Objective  To investigate the value of contrast-enhanced ultrasound (CEUS) in the differential diagnosis of salivary adenoid cystic carcinoma (SACC) and pleomorphic adenoma (PA).

  Methods  A retrospective analysis was performed for patients with SACC or with PA, who attended Department of Oral Surgery, The First Medical Centre of Chinese PLA General Hospital, due to maxillofacial mass from January 2018 to September 2019 and were then pathologically confirmed to have these diseases. Conventional ultrasound and CEUS were performed before surgery to observe the conventional ultrasound features and CEUS enhancement pattern, respectively, and CEUS quantitative analysis software was used to plot the time-intensity curve (TIC) and analyze the parameters.

  Results  Totally 11 patients with SACC and 24 patients with PA were included in this study. Conventional ultrasound showed that there were no significant differences in margin, shape, internal echo, and blood flow between SACC and PA (P >0.05), and a combination of margin, shape, internal echo, and color Doppler flow imaging findings had a sensitivity of 81.8%, a specificity of 58.3%, and an accuracy of 74.3% in the diagnosis of SACC. CEUS showed that among the patients with SACC, 9 (81.8%) showed fast wash-in and unclear boundary after enhancement, while 8 (72.7%) showed enlarged lesion after enhancement; among the patients with PA, 22 (91.7%) showed slow wash-in, 18 (75.0%) showed clear boundary after enhancement, and 22 (91.7%) showed unchanged lesion after enhancement; there were significant differences in enhancement time, enhanced boundary, and extent of lesion after enhancement; a combination of enhancement time, enhanced boundary, and extent of lesion after enhancement had a sensitivity of 90.9%, a specificity of 83.3%, and an accuracy of 88.6% in the diagnosis of SACC. There were no significant differences in enhancement pattern, enhanced ring, enhancement intensity, and enhancement homogeneity between the two groups (P >0.05), and SACC might have the features of margin thickening and an inward vascular structure. The quantitative parameters of TIC showed that compared with PA, SACC had a significantly shorter time to peak (9.97±0.68 s vs 11.77±2.41 s, P <0.05) and a significantly longer time from peak to one half (35.62±4.93 s vs 23.68±7.58 s, P <0.05).

  Conclusion  Contrast-enhanced ultrasound technology can help the differential diagnosis of salivary gland ACC and PA, which has certain clinical value.