Spiradenoma of the breast: a rare diagnostic pitfall in the evaluation of solid-basaloid breast lesions with a dual cell population

Breast spiradenoma is extremely rare, with only 4 cases reported previously. We describe an instructive case of breast spiradenoma resembling adenoid cystic carcinoma (AdCC). A 71-year-old woman underwent excisional biopsy of a breast mass after a conclusive diagnosis was unable to be obtained from core needle biopsy showing an AdCC-like pattern. Histopathologically, the lesion demonstrated solid and cribriform foci comprising basaloid cells, luminal cells, and eosinophilic hyaline material, reminiscent of solid-basaloid AdCC, alongside convoluted lumens, stromal edema, lymphocytic infiltration, and c-kit negativity. On molecular analysis, neither MYB fusion genes nor CYLD gene abnormalities were identified. These results were supportive of spiradenoma. Salivary gland– and skin adnexal–type tumors are challenging to diagnose due to morphological overlaps. This case, highlighting histopathological and molecular features, shows that breast spiradenoma can be a diagnostic pitfall among the differential diagnoses of AdCC.