Spindle cell predominant trichodiscoma: a fibrofolliculoma/trichodiscoma variant considered formerly to be a neurofollicular hamartoma: a clinicopathological and immunohistochemical analysis of 17 cases.

Seventeen solitary nasal tumors that fulfilled all diagnostic criteria of so-called neurofollicular hamartoma, apart from distinct S100-positivity, were compared histopathologically and immunohistochemically with seven typical trichodiscomas from a similar clinical setting. Both the S100-negative neurofollicular hamartoma-like tumors and the trichodiscomas expressed an identical CD13-positive/CD34-positive fibrocytic immunophenotype without co-expression of neural/perineural (S100, neurofilament, epithelial membrane antigen), myogenic (desmin, calponin, muscle-specific actin, and alpha-smooth muscle actin), or melanocytic (S100, HMB45, NKI/C3, MelanA) epitopes. Histopathologically, there was striking morphologic overlap between trichodiscoma and S100-negative neurofollicular hamartoma-like tumor, apart from a highly characteristic fascicularly organized cellular fibrocytic stroma in the latter. We conclude that fibrofolliculoma/trichodiscoma and neurofollicular hamartoma-like tumor are morphologic variants of a single hamartomatous entity in which neurofollicular hamartoma-like tumor occupies the cellular pole of the morphologic spectrum. The entity formerly known as neurofollicular hamartoma appears to be nothing but a particularly cellular trichodiscoma with a distinctively organized stroma composed of CD34-positive fibrocytes. We therefore propose the new term spindle cell predominant trichodiscoma (SCPT) for this particular variant of the morphologic fibrofolliculoma/trichodiscoma spectrum.