Surgical Pathology Criteria

Primary Melanoma of the Anus

Differential Diagnosis

Squamous Carcinoma of the Anus Melanoma of the Anus
May have adjacent squamous carcinoma in situ May have adjacent junctional component
CK5/6 and p63 virtually always positive CK5/6 and p63 negative
S100, HMB45 and MelanA negative S100, HMB45 and/or MelanA positive


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
Rare cases of Paget disease are pigmented

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