Unusual presentation of breast adenoid cystic carcinoma resembling adenomyoepithelioma with multiple distinct foci.

We describe an unusual case of multifocal breast adenoid cystic carcinoma (AdCC) with adenomyoepitheliomatous morphology. Most breast AdCCs are unifocal and only four cases of multifocal AdCC have been reported previously, however, to our best knowledge, multifocality in AdCC confirmed by molecular analysis has not been reported, so this report adds to the literature on this unique presentation. An 80-year-old woman presented with a left breast mass at 1 o'clock and non-mass enhancement lesion at 5 o'clock on imaging. Incisional biopsy at 1 o'clock showed AdCC based on histopathological features and MYB rearrangement by fluorescent in situ hybridization (FISH). As AdCC involved the margins and the non-mass enhancing lesion remained, mastectomy was performed. Microscopically, the lesion at 5 o'clock demonstrated multinodularity and a biphasic epithelial-basaloid/myoepithelial pattern. Although histological features resembled adenomyoepithelioma, MYB rearrangement was identified on FISH, so the 5 o'clock lesion was also diagnosed as AdCC showing an adenomyoepitheliomatous pattern. This unusual presentation is a potential diagnostic pitfall, so pathologists should consider AdCC as a possible differential diagnosis of multifocal basaloid breast tumors with adenomyoepitheliomatous features.