Case Number : Case 2867 - 02 July 2021 Posted By: Dr. Richard Carr

Favour trichoepithelioma

Trichepithelioma/Trichblastoma.

Trichoepithelioma

Trichoepithelioma

trichoepithelioma

Sorry 1st p53 is CK20! As I'm sure you guessed.

Trichoepithelioma

Trichoepithelioma

Lobular Trichoblastoma

Yes this is a trichoepithelioma (or trichoblastoma). This lesion had been reported as a BCC but I was asked to review in view of the young age. The main features of TE/TB at scanning view are the organisation, roughly 50:50 split between stroma and epithelium, rounded profile, lack of surface connections, stromal clefts, condensations of stromal papillary mesenchymal cells (highlighted by a stromal predominant pattern on CD10) and relative lack of prominent palisading, retraction artefact with stromal mucin (can be seen focally in TE/TB). Dendritic (reactive) Merkel cells are prominent in around 1/3rd of cases (not this one). I thought I'd show the pattern for p16, p53 and Ki67 in a benign tumour (on non-sun exposed skin) to compare with the malignant lesions. The p16 has a low level mosaic pattern but as you can see can be constitutively very low in non-sun damaged skin (scalp of a young female). p53 has a low level "wild-type" pattern. Ki67 has hot spots reflecting the mitotically active germinative cells. When I report trichoepithelioma/trichoblastoma if lesions are incompletely excised I normally add a comment that lesions are benign, they may recur and grow to large size but malignant transformation is rare.

This is a great learning case from clinical prospective. In Derm training they indirectly almost imbed in our brain the clinical diagnosis of “cyst or inflamed cyst (if erythematous)” especially for dermal to subcutaneous based lesions on scalp or trunk. You soon realise a host of other diseases can mimic a cyst or inflamed cyst clinically and one has to be vigilant/cautious in such cases particularly in elderly patients.

TB/TE spectrum. good to know the staining pattern, most of the times, H/E diagnosis still is better (the "King"), while IHCs are confusing.