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Breast adenoid cystic carcinoma (ACC) is a primary breast carcinoma that, like salivary gland ACC, displays the t(6;9) translocation resulting in the MYB-NFIB gene fusion and immunopositivity for MYB by immunohistochemistry (IHC). However, it is not well established whether MYB immunoreactivity or rearrangement can be used to support a diagnosis of ACC in a malignant basaloid or benign cribriform breast lesion. Whole sections of primary breast ACC (n=11), collagenous spherulosis (CS; n=7), and microglandular adenosis (MGA; n=5) and tissue microarrays containing 16 basal-like, triple-negative breast carcinomas (TNBC) were labeled for MYB by IHC and underwent MYB fluorescence in situ hybridization using a break-apart probe. Strong, diffuse nuclear MYB labeling was seen in 100% ACC compared with no cases of basal-like TNBC, CS, or MGA (P=0.0001). Any degree of nuclear MYB labeling was seen in 100% ACC compared with 54% of all other cases (P=0.007), with any labeling seen in 71% CS, 63% basal-like TNBC, and 0% MGA. MYB rearrangement was detected in 89% (8/9) of evaluable ACC compared with 4% (1/26) of all other evaluable cases (P=0.0001), with a rearrangement detected in 1 (7%; n=1/15) evaluable basal-like TNBC. Strong, diffuse nuclear labeling for MYB is more sensitive than MYB fluorescence in situ hybridization for breast ACC and can be used to support a diagnosis of ACC in a cribriform or basaloid lesion in the breast. However, weak and focal labeling should be interpreted with caution as it can be seen in other benign cribriform and malignant basaloid lesions.