Solid variant of mammary "adenoid cystic carcinoma with basaloid features" merging with "small cell carcinoma".

We describe a rare case of a solid variant of a mammary adenoid cystic carcinoma with basaloid features (sbACC) and its coexistence with a "small cell" carcinoma (SCC), identified and confirmed by histological and immunohistochemical observations: the absence of glandular structures and PAS-positive globules, positivity for neuroendocrine markers (NSE, synaptophysin and chromogranin), and negativity for 34betaE12 and SMA actin were the aspects suggesting the presence of SCC. Furthermore, positivity for CD10 was found both in sbACC and in SCC, supporting the hypothesis that the two components share the same histogenetic myoepithelial origin and represent an example of dedifferentiation along neuroendocrine phenotype lines occurring in a multipotential neoplastic stem line, already committed towards a myoepithelial phenotype. To our knowledge, this is the first reported case of a solid basaloid adenoid cystic carcinoma merging with an SCC carcinoma. Furthermore, it is the first study in which CD10 was used to investigate the histogenesis of the two neoplastic components.