BACKGROUND: Cylindroma of the breast is a rare benign neoplasm. Since its first description in 2001, 20 cases have been reported in the literature.
RESULTS: We report another case of this rare tumor in a 60-year-old woman with demonstration of the underlying molecular alteration. Histologically, the tumor showed the typical \"jigsaw\" pattern of a dual population of cells with a triple-negative phenotype. The pathognomonic mutation of the CYLD gene mutation was detected by whole exome sequencing. Cylindromas show morphological overlap with the solid-basaloid variant of adenoid cystic carcinoma, which renders this differential diagnosis difficult. However, distinction of these two lesions is of outmost importance, since cylindromas, in contrast to solid-basaloid variant of adenoid cystic carcinoma, behave in an entirely benign fashion.
CONCLUSIONS: Careful evaluation of morphological features such as mitotic figures and cellular atypia is crucial in the diagnostic work-up of triple-negative breast lesions. It is important to keep cylindroma in mind as a pitfall and possible differential diagnosis for the solid-basaloid variant of adenoid cystic carcinoma. Molecular detection of CYLD gene mutation is helpful in cases with ambiguous histology. With this case report, we aim to contribute to a better understanding of mammary cylindroma and facilitate the diagnosis of this rare entity.