Expression of CD117 in primary anorectal malignant melanoma.
Article Type: Letter to the editor
Authors: Laforga, Juan B.
Gasent, Joan M.
Pub Date: 02/01/2009
Publication: Name: Archives of Pathology & Laboratory Medicine Publisher: College of American Pathologists Audience: Academic; Professional Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2009 College of American Pathologists ISSN: 1543-2165
Issue: Date: Feb, 2009 Source Volume: 133 Source Issue: 2
Accession Number: 230151952
Full Text: To the Editor--We read with great interest the article by Guler et al, (1) regarding the expression of melanoma antigens in epithelioid gastrointestinal stromal tumors (GISTs), which are commonly mistaken for melanoma, mainly metastatic melanomas involving the gastrointestinal tract. However, the presence of primary malignant melanomas in the anorectal region, although very rare, is important to recognize because of the possible clinical and treatment implications. Malignant melanomas, most of which are metastatic, occur in up to 15% of primary malignant tumors of the anus. The anorectum is the most common site of primary melanoma in the alimentary tract. (2) Anal melanomas exhibit the same immunohistochemical and ultrastructural features as their cutaneous counterparts. (3) However, similar to melanomas arising in other mucous membranes, anal melanomas are typically acrolentiginous. Invasive melanomas in this region are commonly epithelioid.

We recently studied a case of anal malignant melanoma in a patient, which microscopically showed an epithelioid malignant tumor simulating a GIST. An 82-year-old man complained of rectal bleeding. Physical examination showed a polypoid and firm tumor mass that was thought to be clinically malignant. No endoscopic study was performed. The patient underwent a local transrectal resection of the anorectal polypoid lesion, which was well delimited, whitish, firm, and measured 1.7 cm in diameter. Microscopically, the tumor was located in the submucosa and muscularis mucosae and extended to the rectal mucosa, infiltrating the lamina propria. Extensive areas of ulceration were present. No junctional activity was observed in the squamous lining of the upper mucosa. The tumor cells were epithelioid, with abundant cytoplasm and paranuclear clear spaces. Immunohistochemical study showed strong positivity for CD117, S100 protein, and Melan-A. HMB-45 stain was focally positive at the inner portion of the tumor (20% of the cells). Thus, if our case had been endoscopically biopsied and if a limited immunohistochemical panel of antibodies was used, such as cytokeratins (-), CD34 ([+ or -]), CD117 (+), and HMB-45 (-) (nearly 80% in the main part of the tumor mass), a misdiagnosis of GIST would be likely. Therefore, we favor the application of an adequate immunohistochemical panel in epithelioid tumors of the anorectal region to rule out carcinoma, GIST, or malignant melanoma, which have different biological behaviors. If malignant melanoma is suspected, the panel should include S100 and both the specific antibodies HMB-45 and Melan-A.

Once a diagnosis of malignant melanoma has been established, it must be determined whether the tumor is primary or metastatic. The presence of junctional activity beneath the squamous epithelium indicates a primary malignant melanoma, although that finding may be obscured by ulceration, as it was in our case.

This case illustrates the potential pitfall of relying on a single antibody or inadequate panel of immunohistochemical stains to confirm a diagnosis.

(1.) Guler ML, Daniels JA, Abraham SC, Montgomery EA. Expression of melanoma antigens in epithelioid gastrointestinal stromal tumors: a potential diagnostic pitfall. Arch Pathol Lab Med. 2008;132: 1302-1306.

(2.) Roumen RMH. Anorectal melanoma in the Netherlands: a report of 63 patients. Eur J Surg Oncol. 1996;22:598-601.

(3.) Ben Izhak O, Levy R, Weill S, et al. Anorectal malignant melanoma: a clinicopathologic study, including immunohistochemistry and DNA flow cytometry. Cancer. 1997;79:18-25.

The authors have no relevant financial interest in the products or companies described in this article.

JUAN B. LAFORGA, MD, PhD

Department of Pathology

JOAN M. GASENT, MD, PhD

Department of Oncology

Hospital de Denia

Ptda. Beniadla, S/N

03700 Denia (Alicante)

Spain
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