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Surgical Pathology Criteria
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Primary Melanoma of the Anus

Definition

  • Primary melanoma arising in the anus

Alternate/Historical names

  • Anorectal melanoma

Diagnostic Criteria

  • Histologically identical to cutaneous melanomas
    • Patterns described:
      • Epithelioid
      • Spindle cell
      • Lymphoma-like
      • Pleomorphic
    • About half may be amelanotic
  • Junctional component identifiable in about half of cases
  • Careful analysis of location indicates that the vast majority of anorectal melanomas actually are anal
    • True rectal melanoma is exceedingly rare
  • Metastasis must be ruled out clinically or by identification of a junctional component
  • Poor prognosis
    • Most recurrences are systemic
    • 5-25% 5 year survival
Teri A Longacre MD
Robert V Rouse MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Original posting/updates : 10/7/10, 11/13/11

Supplemental studies

Immunohistology

S100 100%
HMB45 94%
MelanA 93%
Tyrosinase 86%
Vimentin 93%
cKit 75%
Pankeratin 0%
Chute 2006

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

Grading / Staging / Report

  • Melanoma is not graded
  • There is no staging method for anal melanoma
  • It is reasonable to report
    • Thickness
    • Deepest layer involved

Classification / Lists

Anal Tumors and Neoplasms

Extension from rectal lesions must be ruled out

Bibliography

  • Bosman FT, Carneiro F, Hruban RH, Thiese ND (Eds). WHO Classifiication of Tumors of the Digestive System, IARC, Lyon 2010
  • Balachandra B, Marcus V, Jass JR. Poorly differentiated tumours of the anus canus: a diagnostic strategy for the surgical pathologist. Histopathology. 2007 Jan;50(1):163-74.
  • Ragnarsson-Olding BK, Nilsson PJ, Olding LB, Nilsson BR. Primary ano-rectal malignant melanomas within a population-based national patient series in Sweden during 40 years. Acta Oncol. 2008 May 19:1-7.
  • Homsi J, Garrett C. Melanoma of the anus canus: a case series. Dis Colon Rectum. 2007 Jul;50(7):1004-10.
  • Chute DJ, Cousar JB, Mills SE. Anorectal malignant melanoma: morphologic and immunohistochemical features. Am J Clin Pathol. 2006 Jul;126(1):93-100.
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