Anus & perianal area

Other tumors

Melanoma



Last author update: 5 December 2023
Last staff update: 5 December 2023

Copyright: 2002-2024, PathologyOutlines.com, Inc.

PubMed Search: Anal melanoma

Olivia A. Sagan, M.D.
Aaron R. Huber, D.O.
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Cite this page: Sagan OA, Huber AR. Melanoma. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/anusmelanoma.html. Accessed March 29th, 2024.
Definition / general
  • Rare primary malignant melanocytic neoplasm of the anal mucosa
Essential features
  • Rare primary malignant melanocytic neoplasm of the anal mucosa, most common in White and female patients in the sixth decade of life
  • Biopsy is necessary for diagnosis with positive melanocytic markers (SOX10, S100, HMB45, MelanA, etc.)
  • Metastasis from another location must be ruled out before diagnosing primary anal melanoma
  • There is currently no consensus on how to pathologically stage anal melanoma
  • Patients have a poor prognosis due to advanced disease on presentation and aggressiveness of the tumor
Terminology
  • Primary anal melanoma
  • Anorectal melanoma
  • Mucosal lentiginous melanoma
ICD coding
  • ICD-10
    • C21.0 - malignant neoplasm of anus, unspecified
    • C21.1 - malignant neoplasm of anal canal
    • C21.8 - malignant neoplasm of overlapping sites of rectum, anus and anal canal
Epidemiology
Sites
Pathophysiology
Etiology
  • Melanocytes are normally found at the anal transition zone and squamous zone (Clin Colon Rectal Surg 2011;24:171)
  • Mucosal melanomas, including anal melanoma, are not associated with ultraviolet (UV) radiation exposure, unlike cutaneous melanomas (Nature 2017;545:175)
  • Otherwise, the etiology of anal melanoma is not well established
Clinical features
  • Symptoms are typically vague, including anorectal bleeding, pain, change in bowel habits and sometimes a mass, which can be confused with other conditions (e.g., hemorrhoids) and may lead to delayed diagnosis (Surg Clin North Am 2020;100:629)
  • May be discovered as an incidental finding in a hemorrhoidectomy or anal polyp resection specimen (Surg Oncol Clin N Am 2017;26:143)
  • Patients often have delayed presentation or are misdiagnosed, which results in advanced disease, often with local lymph node metastasis (Surg Clin North Am 2020;100:629)
  • Some association with human immunodeficiency virus (HIV) (Surg Oncol Clin N Am 2017;26:143)
Diagnosis
Laboratory
  • No specific findings for this entity
Radiology description
  • MRI is preferred due to higher soft tissue resolution
  • Melanin pigmented lesions typically demonstrate high signal intensity on T1 weighted imaging and mixed signal intensity on T2 weighted imaging
  • It is common for amelanotic lesions to not display these characteristic findings on MRI
  • Reference: BMJ Case Rep 2021;14:e247421
Radiology images

Images hosted on other servers:
Anorectal mass (CT of pelvis)

Anorectal mass (CT of pelvis)

Anorectal mass (MRI)

Anorectal mass (MRI)

Anorectal mass (MRI / PET)

Anorectal mass (MRI / PET)

Prognostic factors
Case reports
Treatment
Clinical images

Images hosted on other servers:
Anorectal mass

Anorectal mass

Prolapsed anal mass

Prolapsed anal mass

Prolapsed anorectal mass

Prolapsed anorectal mass

Polypoid lesion on colonoscopy

Polypoid lesion on colonoscopy

Gross description
  • Lesions can be varied in their presentation, including size, shape and color
  • Usually present as large, expansive, nodular masses
  • Majority of lesions are pigmented but can be nonpigmented
  • Ulceration is common
  • Reference: Surg Clin North Am 2020;100:629
Gross images

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Pigmented anal mass

Pigmented anal mass

Anorectal mass

Anorectal mass

Frozen section description
  • Frozen section is not recommended
Microscopic (histologic) description
  • Histomorphology can vary greatly but solid sheets of atypical epithelioid cells originating from the epithelium is the most common presentation; however, the cytomorphology may be composed of spindled, pleomorphic or small round blue cells while the architectural patterns may be nested, pseudopapillary or pseudoalveolar (Hum Pathol 2018;79:77)
  • Pigmentation is variable and can be absent in up to 40% of cases, which is much more common than in cutaneous melanoma (< 5% amelanotic) (Hum Pathol 2018;79:77, Surg Oncol Clin N Am 2017;26:143)
  • Ulceration can be present
  • Identification of a junctional component adjacent to the invasive tumor can support the diagnosis of primary anal melanoma (Surg Oncol Clin N Am 2017;26:143)
  • Melanoma, including anorectal examples, may undergo dedifferentiation which is defined as a tumor that has lost evidence of melanocytic differentiation (morphologic and immunophenotypic) (Int J Surg Pathol 2019;27:923)
    • These dedifferentiated examples may aberrantly express various nonmelanocytic markers
    • Dedifferentiation is rare but is seen more commonly in metastases of cutaneous origin
Microscopic (histologic) images

Contributed by Aaron R. Huber, D.O. and @RaulSGonzalezMD on Twitter
Tumor underlying squamous epithelium

Tumor underlying squamous epithelium

Epithelioid and Spindled Cells

Epithelioid and spindled cells

Malignant cells with pigment

Malignant cells with pigment

Ulcerated mucosa

Ulcerated mucosa

Malignant cells with pigment

Malignant cells with pigment

Large nuclei with prominent nucleoli

Large nuclei with prominent nucleoli


SOX10

SOX10

MelanA

MelanA

PRAME

PRAME

Primary anal melanoma Primary anal melanoma Primary anal melanoma

Primary anal melanoma

Virtual slides

Images hosted on other servers:
Malignant infiltrative epithelioid cells

Malignant infiltrative epithelioid cells

MelanA

MelanA

S100

S100

HMB45

HMB45

Pankeratin

Pankeratin

Cytology description
  • FNA specimens are usually obtained from metastases
  • Largely dispersed population of pleomorphic epithelioid cells or spindle cells
  • Nuclear pleomorphism and prominent nucleoli
  • Eccentric nuclei with pseudoinclusions can be seen
  • Cytoplasmic melanin is seen as fine yellow-brown or blue granules, depending on the slide preparation
  • May have cells with double image mirror nuclei (demons)
  • References: Am J Clin Pathol 2007;127:385, Int J Clin Exp Pathol 2010;3:367, Cancer Cytopathol 2022;130:18
Cytology images

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Metastatic melanoma smears Metastatic melanoma smears

Metastatic melanoma smears

Negative stains
Molecular / cytogenetics description
Sample pathology report
  • Anus, pigmented lesion, excision:
    • Malignant melanoma (see comment)
    • Margins are free of involvement
    • Comment: Immunohistochemical staining was performed and the neoplastic cells showed positivity for SOX10 and PRAME, while they were negative for pancytokeratin. The immunophenotypic findings support the diagnosis of primary anal melanoma.
Differential diagnosis
Board review style question #1

A 66 year old White woman presents with anal pain and is found to have a pigmented anal mass. The mass is biopsied and the histology is depicted in the H&E image shown above. Which immunohistochemical stain would best support the correct diagnosis?

  1. CD45
  2. CDX2
  3. HMB45
  4. Pancytokeratin
Board review style answer #1
C. HMB45. The H&E image demonstrates anal melanoma. HMB45 is a melanocytic marker that would confirm the diagnosis of melanoma, along with the malignant features seen on H&E. Answer A is incorrect because CD45 is a lymphocytic marker. CD45 should be ordered to rule out lymphoma but a negative result would not confirm the diagnosis in this case. Answer D is incorrect because melanoma is typically pancytokeratin negative. This would be a good confirmatory stain but would not be diagnostic of melanoma by itself. Answer B is incorrect because CDX2 is a common marker for colonic adenocarcinomas. CDX2 should be used to rule out a primary rectal adenocarcinoma that involves the anus but will be negative for any anal malignancies, including melanoma.

Comment Here

Reference: Anal melanoma
Board review style question #2
Which of the following characteristics is similar for both cutaneous and anal melanoma?

  1. Pathologic staging
  2. Presentation
  3. Prognosis
  4. Staining / IHC
Board review style answer #2
D. Staining / IHC. Cutaneous and anal melanoma will stain with similar melanocytic markers (SOX10, S100, HMB45, MelanA, etc.). Answer C is incorrect because anal melanoma has a worse prognosis when compared to cutaneous melanoma, especially because patients with anal melanoma often have advanced disease on presentation due to the vague clinical symptoms and difficulty in diagnosis. Answer B is incorrect because anal melanoma typically presents with vague symptoms and not the typical ABCDE (asymmetry, border, color, diameter, evolving) criteria for cutaneous melanoma. Answer A is incorrect because anal melanoma does not have a standardized pathologic staging system. Anal melanomas can be staged similarly to cutaneous melanoma but this is not universal.

Comment Here

Reference: Anal melanoma
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