Case Number : Case 2581 - 28 May 2020 Posted By: Saleem Taibjee

consistent with GA. DD: annular erythema. 

GA, interstitial type. 

Clue: It is not GA, but a mimick

interstitial granulomatous dermatitis?

Clinically there are erythematous to slightly violaceous dermal arcuate and annular plaques with a striking well defined border on his back.

Some of the interstitial cells are large with an irregular outline.

I wonder whether it is worth considering further IHC to clarify the nature of those cell especially if lymphoid or not? The appearance of the rash is not consistent with GA commonly seen.

An IHC would be prudent to define the clonality of interstitial cells, they appear to be lymphoid, with large nuclei. I think of some type of lymphoma / leukemia infiltrating the skin.

Deep variety of erythema annulare centrifugum.

This was a rather intriguing case which I encountered in routine digital sign out. I nearly fell into the trap of concurring with clinician and opting for GA (interstitial variant) because it is a remarkably good mimick at low/medium power. But fortunately I dwelt for a moment on the cytology, and noted that the interstitial cells appeared to have large nuclei, and were not right for histiocytes.

A few of the stains are shown below, but in summary, the staining of the intersitial cells:

interesting sir

Very interesting case! Thanks Saleem for sharing!