MYB expression by immunohistochemistry is highly specific and sensitive for detection of solid variant of adenoid cystic carcinoma of the breast among all triple‐negative breast cancers

Background Adenoid cystic carcinoma is a rare subtype of triple‐negative breast carcinoma. These low‐grade tumours, which are treated by simple mastectomy and have an excellent prognosis compared to other triple‐negative breast carcinomas. Solid‐variant adenoid cystic carcinomas have basaloid features and are difficult to distinguish morphologically from other triple‐negative breast cancers. Breast adenoid cystic carcinoma exhibits MYB protein overexpression, which can be detected by immunohistochemistry (IHC). Aim We compared the IHC expression of MYB in solid‐variant adenoid cystic carcinoma with that in other triple‐negative breast cancers. Methods We conducted IHC staining of 210 samples of triple‐negative breast cancers, including solid‐variant adenoid cystic carcinoma (n = 17), metaplastic breast carcinoma (n = 44), basaloid triple‐negative breast cancer (n = 21), and other triple‐negative invasive ductal carcinoma (n = 128). We classified nuclear staining of MYB as diffuse/strong (3+), focal moderate (2+), focal weak (1+), or none (0). Results All 17 solid/basaloid adenoid cystic carcinoma cases exhibited 3+ MYB expression. Of the 21 solid/basaloid triple‐negative breast cancers, one (5%) had 2+ expression, seven (33%) 1+ expression, and 13 (62%) 0 expression. Of the 44 metaplastic carcinoma cases, 39 cases (89%) had no (0) staining, and the other five cases had focal weak (1+) or moderate (2+) staining. Among the 128 triple‐negative invasive ductal carcinoma cases, 92 cases (72%) had no (0) staining, 36 cases (28%) exhibited focal weak (1+) or moderate (2+) staining. Conclusions Our study revealed diffuse/strong MYB staining (3+) only in solid/basaloid adenoid cystic carcinomas. Thus, we recommend routine MYB IHC staining in triple‐negative breast carcinoma with solid/basaloid morphology to improve diagnostic accuracy. Solid variant adenoid cystic carcinoma of the breast shows a more favourable prognosis than other triple‐negative breast cancers. MYB immunohistochemistry is a reliable method for distinguishing this variant, exhibiting strong positivity in solid adenoid cystic carcinoma, in contrast to the negativity or diffuse positivity seen in other triple‐negative breast cancers.