Case Number : Case 988 - 7th April Posted By: Guest

Pilar sheath acanthoma.

Pilar sheath acanthoma

Lobular infundibuloisthmicoma, just to bring an alternative name [url="http://journals.lww.com/amjdermatopathology/Citation/1996/08000/Pilar_Sheath_Acanthoma__Lobular.24.aspx"]proposed by Dr. Hurt[/url].

Pilar sheath acanthoma, interesting alternative name !!!

Agree..

The outer root sheath passes from the lower outer bulb, up around the entire follicular stem and up around the isthmus and blends imperceptibly with the infundibulum. You could simply argue tricholemmoma (common) and pilar sheath acanthoma (quite uncommon) are morphological variants (with actually more in common than not - I note this case is from the upper lip a typcial site for tricholemmoma) of "outer root sheath-oma". Whether the infundibular component (seen in both tumours in my view) is reactive or neoplastic I do not know but the cellular "boundaries" between isthmus and infundibulum are not perceptible in normal follicles, cysts (we have all seen examples of hybrid epidermal - infundibular - and pilar - outer root sheath of the isthmus/upper stem - cysts) and neoplasms like tricholemmoma and especially the common (infundibular-tricholemmal) follicular SCC where most tumours have both components (contrary to some literature implying differently).

Turns out I have not but I have only diagnosed 5 typical pilar sheath acanthomas in nearly 15 years at Warwick (compared with probably >100 tricholemmoma). I did have 3 other cases that were a little interesting but were morphologically overlapping with trichoepithelioma or trichofolliculoma. Having said that if any had been CD34 positive I might have called them tricholemmoma anyway although I don't appear to have recorded them.