MYB immunostaining is a useful ancillary test for distinguishing adenoid cystic carcinoma from pleomorphic adenoma in fine-needle aspiration biopsy specimens

Background: The distinction between adenoid cystic carcinoma (ACC) and pleomorphic adenoma (PA) on fine-needle aspiration biopsy (FNAB) can be challenging. Recently, a specific translocation t(6;9) involving the v-myb avian myeloblastosis viral oncogene homolog (MYB) and nuclear factor I/B (NFIB) genes was identified in ACCs, in which it contributes to MYB overexpression. The authors investigated the use of MYB immunocytochemistry in FNAB specimens as an ancillary test for the cytologic diagnosis of ACC.

Methods: The expression of MYB was assessed in alcohol-fixed cytologic smears of histologically confirmed ACCs (n = 20) and PAs (n = 20) using immunocytochemistry. The corresponding ACCs and PAs from formalin-fixed, paraffin embedded surgical resection specimens also were stained immunohistochemically for MYB. The nuclear expression of MYB was assessed semiquantitatively using a scoring system (from 0 to 6) that combined the proportion and the intensity of staining. A score ≥4 was considered positive, and a score ≤3 was considered negative.

Results: On FNAB material, 80% of ACCs (N = 16 of 20) were immunocytochemically positive for MYB. In contrast, all PAs (N = 20 of 20) were negative for MYB (P < .0001). The sensitivity of MYB on FNAB was 80%, and the specificity was 100% relative to PA. Results on corresponding histology were similar to the cytology results; however, there was often a zonal staining pattern with central areas of tumor (range, 20%-90%) that lacked immunoreactivity, thus suggesting that the immunoreactivity was maintained better in alcohol-fixed FNAB material.

Conclusions: In contrast to PAs, a majority of ACCs are immunocytochemically positive for MYB. The test is more effective using alcohol-fixed FNAB material and is potentially useful for the cytologic distinction of ACC and PA.