Extramammary Paget Disease

Differential Diagnosis

Pagetoid Squamous Cell Carcinoma In Situ / HSIL	Extramammary Paget Disease
Atypical cells usually merge with surrounding keratinocytes	Discrete population of atypical cells
Atypical cells may keratinize	Atypical cells may form lumens or be mucin positive
Desmosomes and keratohyaline granules may be visible in atypical cells	Desmosomes and keratohyaline granules not present
HMWCK+, p63+, CK7 neg, BerEp4 neg	CK7 >90%+, p63 & HMWCK neg (positive if underlying urothelial carcinoma), BerEp4+
GCDFP15 neg	GCDFP15 positive if primary, variable if secondary

Melanoma In Situ of the Anus	Extramammary Paget Disease
Substantial proportion of atypical cells sit directly on the epidermal basement membrane	Atypical cells at all levels but frequently have at least one layer of normal cells separating them from the basement membrane
No lumen formation or mucin positivity	Atypical cells may form lumens or be mucin positive
CK7 and other keratins negative	CK7 and other broad spectrum antikeratins >95%+
HMB45 and MelanA 60-95%+	HMB45 and MelanA negative