anorectal melanomas ( arm ) are uncommon neoplasias , and most of them are metastases from a cutaneous primary . primary arm represents 0.4 to 1.6% of all melanoma manifestations [ 2 , 3 ] . nevertheless , the anorectum is the most common site of primary melanoma in the alimentary tract . malignant melanoma ( mm ) occurs in up to 15% of primary malignant tumors of the anus . the tumor originates from melanocytes that are normally found in the transitional zone above the dentate line . in early stages grossly arm may display a polypoid mass covered by a smooth surface which may eventually be ulcerated and usually extends into the lower rectum . the epidemiology of arm has not been studied , although its putative relevance and prognostic studies have been based on referred cases . some epidemiologic data suggest that immunodeficiency virus infection increases the risk of arm [ 3 , 7 ] . due to its low incidence , treatment of arm is not well standardized since it was implemented based on retrospective studies referring to a short series of cases from data collected for up to 64 years . arm presents as a large protuberant or ulcerating mass which usually extends into the lower rectum . most patients present with lesions > 2 mm thick . at the time of diagnosis about 30% of cases present distant metastases , and metastases eventually develop in 60% an 82-year - old man complained of rectal bleeding . physical examination showed a polypoid and firm tumor mass which was thought to be clinically malignant . no endoscopic study was performed . chest x - ray and whole - body computed tomography failed to show any tumor mass . the gross specimen was a segmental anorectal resection measuring 2.6  1.7  0.7 cm . in the center , a polypoid well - delimitated lesion , measuring 1.7 cm in diameter , which was whitish and elastic , was observed . microscopically , the tumor was located in the submucosa and muscularis mucosae and extended to rectal mucosa infiltrating lamina propria . mitotic figures were frequent and atypical , averaged 16 per 10 high - power fields . vascular or perineural invasion was not observed . the tumor cells showed strong positivity for vimentin ( v9 ) , cd117 ( c - kit ) , s-100 protein , and mart-1 ( melan - a ) . stain was focally positive at the inner portion of the tumor ( 20% of the cells ) . stains for cytokeratins , smooth muscle actin , desmin , h - caldesmon , chromogranin , synaptophysin and cd34 ( qben-10 ) were negative . with ki67 a diagnosis of submucosal amelanotic malignant melanoma , ulcerated of the anus , breslow 2.8 mm thick , was made . after the diagnosis , the patient was examined by a dermatologist and no skin lesion suggestive of cutaneous melanoma was observed . currently , 5 months after the diagnosis , the patient is well and free of tumor . the gross specimen was a segmental anorectal resection measuring 2.6  1.7  0.7 cm . in the center , a polypoid well - delimitated lesion , measuring 1.7 cm in diameter , which was whitish and elastic , was observed . microscopically , the tumor was located in the submucosa and muscularis mucosae and extended to rectal mucosa infiltrating lamina propria . mitotic figures were frequent and atypical , averaged 16 per 10 high - power fields . vascular or perineural invasion was not observed . the tumor cells showed strong positivity for vimentin ( v9 ) , cd117 ( c - kit ) , s-100 protein , and mart-1 ( melan - a ) . stain was focally positive at the inner portion of the tumor ( 20% of the cells ) . stains for cytokeratins , smooth muscle actin , desmin , h - caldesmon , chromogranin , synaptophysin and cd34 ( qben-10 ) were negative . with ki67 a diagnosis of submucosal amelanotic malignant melanoma , ulcerated of the anus , breslow 2.8 mm thick , was made . after the diagnosis , the patient was examined by a dermatologist and no skin lesion suggestive of cutaneous melanoma was observed . currently , 5 months after the diagnosis , the patient is well and free of tumor . however , similar to melanomas arising in other mucous membranes , they are typical of the acrolentiginous type . invasive melanomas in this region are commonly epithelioid , although spindle cell and desmoplastic variants have been described . differentiation of anal malignant amelanotic epithelioid melanoma from paget 's disease , lymphoma , undifferentiated carcinoma and gastrointestinal stromal tumor ( gist ) can be difficult on the basis of histologic criteria alone . the microscopic appearance should indicate a malignant epithelioid gist , with high risk of metastasis because of the presence of nuclear atypia and large number of mitosis / hpf . although it is infrequent to find a high - grade gist measuring less than 2 cm , it could be possible . rectal gists are nearly always positive for cd34 . unlike leiomyomas and leiomyosarcomas , colorectal gists rarely present as mucosal polyps because they arise within the muscularis propria [ 12 , 13 ] . besides , those muscular tumors stain positively with smooth muscle antigens ( desmin and caldesmon ) , all of which were negative in our case . it should be kept in mind that c - kit positivity is not specific of gist . in fact it is well - known that leukemias , some lymphomas , germinal cell tumors , and melanomas show positive results with this antibody , and awareness of this is important in order to prevent a diagnostic error with treatment and outcome consequences . chute el al . studied a series of 17 cases of arm and reported that c - kit is frequently expressed ( 75% ) , although specific melanoma markers hmb-45 and melan - a are not always expressed in this tumor . on the other hand , melanomas may not always uniformly stain for hmb-45 . it should be realized that mart-1 ( melan - a ) may be positive in cases of melanomas which completely lack reactivity or show a heterogeneous pattern of reactivity with hmb-45 stain , as in the case discussed here . recently , seya et al .   reported a rectal malignant melanoma that was misdiagnosed in the endoscopic biopsy as neuroendocrine carcinoma or gist because of the reactivity with cd56 , s100 and cd117 , and a malignant melanoma was ruled out because hmb-45 was negative . however , the study of the surgical specimen showed hmb-45 and mart-1 reactivity in both biopsy and surgical specimen . similarly , if our case had been endoscopically biopsied and if a limited immunohistochemical panel of antibodies would have been employed , such as cytokeratins- , cd34-/+ , ckit+ and hmb-45- ( in the main part of the tumor mass , nearly 80% ) , a misdiagnosis of gist would have been more likely . therefore , we favor the application of an adequate immunohistochemical panel in epithelioid tumors of the anorectal region in order to rule out leiomyosarcoma , gist or mm , which pursue different biological behaviors . in the case of suspected malignant melanoma , this panel should include s-100 , vimentin and both specific antibodies hmb-45 and mart-1 . 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 favors a primary malignant melanoma , although it may be obscured by ulceration . in fact , our case showed large areas of ulceration which could miss this important key feature . however , the absence of the obviously helpful history of primary melanoma elsewhere should it exist ; we assumed confidently that the tumor was primarily from the anorectal region . the prognosis of anal melanoma is poor , regardless of the type of treatment [ 7 , 10 , 19 ] , with a mean 5-year survival of approximately 15% [ 3 , 20 ] , ranging from 6 to 31% [ 21 , 22 ] . thus , arm may represent a systemic disease at the time of diagnosis in most cases , and the choice of surgical or other local procedures to amend the primary tumor may not have great influence on the systemic course of the disease . most recurrences occur within the first 30 months , and common sites are the inguinal lymph nodes , lung , liver and bones . the thickness of the tumor measured from the top of the overlying intact mucosa or ulcerated tumor correlates with the outcome . anal melanomas of less than 2.0 mm in thickness have better prognosis than those greater than 2.0 mm in thickness . regarding the surgical treatment of arm , it is a matter of debate between wide local excision ( wle ) and abdominoperineal resection ( apr ) . a meta - analysis which included compiled data of 426 patients failed to show the advantage of either approach with respect to the overall survival . it should be noted , however , that in most studies it was difficult to compare patients prognostic factors due to the fact that the information provided was insufficient . nevertheless , there was a trend for local disease to be more effectively controlled by apr than wle [ 4 , 10 , 19 , 23 ] . reported that adjuvant radiation of the pelvis and inguinal lymph nodes after wle showed similar rates of local tumor control as apr . wle should be the procedure of choice for other investigators , because the alternative of apr may have a high mortality rate and lack of survival advantages . in arm treatment , therapeutic strategies reported a correlation between overall survival and the size of the tumor . among 33 patients , only 2 were long - term survivors and the tumor was less than 2 cm in diameter . unfortunately , the histopathologic data were not provided for all patients . on the other hand , slingluff et al .   suggested that the only significant value for survival was the stage of disease at the time of diagnosis , whereas ballo et al . reported a favorable disease - free survival with less than 4 mm of tumor thickness . 9 out of 19 patients presented either regional or distant metastases at the time of diagnosis . the incidence of local recurrences was lower following apr even for patients with less favorable tumors . the authors concluded that wle alone is not sufficient for local tumor control in thick arm .
epithelioid cell tumors presenting in the gastrointestinal tract are uncommon , but when they arise , arriving at a correct diagnosis is important . 
 we report a case of anal malignant melanoma in an 82-year - old man who microscopically showed an epithelioid malignant tumor simulating a gastrointestinal stromal tumor . 
 c - kit stain and melan - a were diffusely and strongly positive , while hmb-45 was focally positive . 
 this case illustrates the potential pitfall of relying on a single antibody or inadequate panel of immunohistochemical stains to confirm the diagnosis . 
 we recommend to apply an adequate immunohistochemical panel which includes s-100 protein , hmb-45 and melan - a in order to make an accurate diagnosis , and discuss the differential diagnosis and surgical treatment modalities .