Case Number : Case 2538 - 30 March 2020 Posted By: Iskander H. Chaudhry

Poroid neoplasm (probably poroma) with squamous differentiation.

I see trichilemmal and matrichal follicular differentiation. Favor a desmoplastic trichilemmoma, with basaloid follicular differentiation

Dr Chaudhry - See below:

Skin with a lesion marked by the expansion of the follicular infundibulum over a number of follicles. The component cells are in part small and basaloid and in part squamoid. The distended infundibulum has a minor sebaceous component as well as a few rudimentary ducts some of which are enlarged and distorted to form squamous cysts. Nuclear palisading is not very apparent. Mitoses are not a feature and there are no apoptotic figures and no necrosis. These expanded lobules are surrounded by a plasma cell infiltrate.

The lesion is diffusely negative for CD34 that also shows no specialized stroma and highlights delicate vasculature. Immunochemistry for BerEP4 is diffusely negative. PRAME shows only staining of mature sebocytes only and weak nuclear staining in the central more squamoid lobule. EMA shows plasma cell staining and staining in the
more squamous areas. The lesion is close to the deep resection margin. This is an in situ lesion of infundibular origin.

The presence of cells with sebaceous differentiation raises the possibility of sebaceoma with a central desmoplastic trichilemmoma despite the lack of CD34.