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
