Case Number : Case 2556 - 23 April 2020 Posted By: Saleem Taibjee

Irritated Seborrheic Keratosis.

Proliferated trichilemmal cyst

The lining of the cystic lesion has a granular layer so I think it's not outer root sheath differentiation (proliferating trichilemmal cysts usual display outer roots sheath differentiation according to the newest WHO blue book , meaning abrupt keratinization without granular layer). Some might just call the lesion a well-differentiated keratinizing SCC and be done with it. I think if someone wants to call this proliferating variant of epidermal (infundibular) cyst (so-called proliferating epithelial cyst) because of the presence of the granular layer, I would agree too. 

Inverted follicular keratosis

I also consider this inverted follicular keratosis.

agree Inverted follicular keratosis of helwig

Interesting case. While features such as squamous eddies are compatible with IFK, the overall crateriform architecture makes me wonder for keratoacanthoma. A keratoacanthoma in a young patient raises the consideration for genetic syndromes such as Muir-Torre.

Thanks for the interesting comments. I concluded that this is an example of proliferating epidermoid cyst. McKee's Pathology of the Skin states that this is rare, more common in males, anatomical sites of predilection including pelvic area, scalp and trunk. In some instances it is longstanding suggesting that it may arise within a pre-existing epidermoid cyst. Recurrence rate is 20%, but there are no reports of metastasis.

I could not ignore the cystic architecture in this case. Regarding histology, the cyst wall shows a granular layer with epidermal/infundibular keratinisation. Interestingly, the proliferative component may vary in this entity, but notably may show conspicuous squamous eddies (as in this case) reminiscent of inverted follicular keratosis. In other examples, the proliferative component may be multicystic, keratotic or verrucous.

I have alerted the clinician of the risk of local recurrence in this case.

I'm a bit late to the party but I'd go for an unusual cystic IFK with infundibular and tricholemmal differentiation. Nice mucin giving rise to pseudoacantholysis. I think I've seen a similar one in the recent past - not a common manifestation.