Case 1
Clinical history: 50 year old man with an ulcerated papule on the nasal ala. 
Diagnosis: Desmoplastic trichilemmoma
Video explanation: https://kikoxp.com/posts/8362
 
Discussion: 
 
Excerpt from my book, Survival Guide to Dermatopathology, used with permission from the publisher (Innovative Science Press):
 
Trichilemmoma
 
This benign follicular proliferation is quite common. It presents in adults as a small papule on the nose/face, often clinically resembling a verruca (some believe it may actually be a variant of verruca rather than a follicular neoplasm). It also has a verrucous appearance microscopically, with papillomatosis, hypergranulosis, tiers of parakeratosis, and dilated papillary dermal vessels. Below this verrucous surface, the epidermis expands and bulges/pushes down into the upper to mid dermis forming a bowl shape. The keratinocytes in this bulging/pushing area usually show pale/clear cytoplasm, and the basal layer shows prominent nuclear palisading, at least focally; these features are recapitulating the outer root sheath of a normal hair follicle. Dense pink basement membrane deposits may be present in the dermis. 
 
Desmoplastic trichilemmoma is a benign morphologic variant of trichilemmoma that has a central zone with cords of basaloid/squamoid cells arranged haphazardly in a desmoplastic and/or myxoid/mucinous stroma.  These areas of desmoplastic change can closely resemble infiltrative BCC or SCC, especially on a small biopsy. Identifying adjacent conventional trichilemmoma (clear/pale cells and peripheral palisading), which is usually present at the periphery of the lesion, is the key to making the correct diagnosis and avoiding unnecessary additional surgery for the patient. However, on a small superficial biopsy, these areas may be absent, making the distinction nearly impossible. The desmoplastic pattern in trichilemmoma is a potential pitfall for pathologists but is of no clinical significance; these are totally benign. When there is uncertainty on a small biopsy between desmoplastic trichilemmoma and infiltrative BCC/SCC, I use a term like “basaloid neoplasm” and a comment to explain the uncertainty and give a differential diagnosis.  
 
Patients with Cowden syndrome (an autosomal dominant disease due to germline PTEN mutation) develop multiple trichilemmomas and have increased risk for visceral carcinomas (including thyroid, breast, and endometrial). My former dermatology residents taught me a fun way to remember this: Cowden syndrome is associated with “trichilem-mooo-mas” (a cow says “mooo”). The vast majority of trichilemmomas seen in my practice are solitary and sporadic rather than syndromic. I do not routinely mention Cowden syndrome in my report unless there are multiple trichilemmomas or some other history suggestive of Cowden syndrome. 
 
References:
 
·      Survival Guide to Dermatopathology. JM Gardner. Innovative Science Press. 2019. 
·      Fulton EH, Kaley J, Gardner JM. Skin adnexal tumors in plain language: a practical approach for the general surgical pathologist. Arch Pathol Lab Med. 2019 Jul;143(7):832-851
 
Illustration (use arrows to see 5 still images for this case):
1-1.         The acanthotic epidermis bulges/pushes down into the dermis. The surface is ulcerated. There are organized pale/clear cells at the periphery but disorganized basaloid to squamoid cells in the center. 
1-2.         A closer view of the central area shows pseudo-infiltrative cords of basaloid cells with desmoplastic stromal change. The appearance closely mimics infiltrative pattern basal cell carcinoma. 
1-3.         Squamous metaplasia is often present, which can mimic infiltrative squamous cell carcinoma. The growth pattern is concerning, but there is minimal cytologic atypia. 
1-4.         The key to the diagnosis is recognizing classic features of conventional trichilemmoma at the periphery: clear or pale cells, peripheral palisading, and a smooth pushing border. 
1-5.         The peripheral palisading recapitulates the outer root sheath of a normal hair follicle.