**5. Histopathology**

As none of the clinical features are unique to AM, histological examination of the suspected lesions should be performed for the definitive diagnosis of AM. The main cytological features of AM are highly cellular smear, presence of binucleated or multinucleated cells, and cytoplasmic melanin pigment. However, melanin pigment is found in only about 27% cases [29].

On gross examination, the lesion appears as polypoidal or ulcerated lesion with or without brown-black pigmentation (**Figure 3**). The histological description includes cell type, degree of melanin pigmentation and mitotic index (Ki-67) [20]. Typically, AM consists of spindle-shaped (**Figure 4**) or epitheloid cells with high nuclear pleomorphism and presence of cytoplasmic melanin granules (**Figure 5**) [30]. About 20% cases are truly amelanotic on histology [31]. Four subtypes of AM based on histology are epitheloid, spindle cell, lymphoma-like and pleomorphic [32]. In the absence of melanin pigments, the tumor morphology can mimic lymphoma and gastrointestinal stromal tumors. Immunohistochemistry helps in differentiate AM from other tumors. Melanoma antigens such as S-100, HMB-45 and vimentin are positive in 78, 94 and 100% cases (**Figure 6**) [31]. The characteristic

#### **Figure 3.**

*Gross examination of the specimen after abdominoperineal resection showing the pigmented polypoidal hard growth (arrow) of about 3 cm reaching up to the outer surface (A) and involving the anorectal junction (B).*

**55**

*Anorectal Melanoma*

**Figure 4.**

**Figure 5.**

**Figure 6.**

*and SOX 10 (C).*

*DOI: http://dx.doi.org/10.5772/intechopen.93759*

marker of gastrointestinal stromal tumor, c-Kit is positive in about three-fourth cases of AM (**Figure 4**) [32]. In some cases, the tumor cells may show positivity for CEA, CD30 and CD68 similar to colorectal adenocarcinoma and other tumors [33]. Hence, a panel of markers should be tested to confirm the diagnosis of AM in doubtful cases. Some unique markers for melanoma with high specificity and low sensitivity include Melanin A, Mart-1 antibodies [20]. Interestingly, Ki-67 and proliferating cell nuclear antigen (PCNA) immunostains have been found to

*Immunohistochemical analysis of the anorectal melanoma showing positivity for HMB 45 (A), Melan A (B)* 

*Microscopic examination of the tumor showing diffuse infiltration of the anorectal region by large epitheloid* 

*cells with vesicular and prominent nuclei (A) H&E x 10 and (B) H&E x 40.*

*Histological examination of anorectal melanoma showing the diffuse infiltration of the tissue by spindleshaped cells with dense eosinophilic cytoplasm and pleomorphic nuclei: (A) H&E x10, (B) H&E x 20, and (C) H&E x 40. Immunohistochemical analysis revealed positive staining with c-KIT (D) and Melan A (E).*

Tumor-infiltrating lymphocytes (TILs) provide a reflection of the tumor microenvironment. Presence of TILs in high concentration is associated with high programmed cell death protein 1 (PD-1) [34]. High PD-1 expression indicates better prognosis for patients with AM due to good response to targeted therapies. TILs can be seen on hematoxylin and eosin stains and also with immunohistochemistry. The majority of these TILs are CD8-positive cells. In a study of 43 AM patients, TILs

predict survival in patients with AM [31].

were present in 55% cases [35].

#### **Figure 4.**

*Melanoma*

sues due to high metabolic rate [20].

appear hypoechoic with uneven internal echoes [28].

ment is found in only about 27% cases [29].

**4.3 Endoscopic studies**

**5. Histopathology**

as well as for the detection of metastatic lesions in the liver (**Figure 2**) [27]. PETCT is the recommended imaging for the staging and response assessment of metastatic melanoma [20]. Melanoma cells have higher FDG avidity compared to normal tis-

For deeply located AM, especially rectal melanoma, endoscopy is very useful to visualize the lesions and take biopsies for histological examination. On endoscopy, the lesions appear as black or brownish plaques, ulcers or polyps due to the melanin pigment. The accuracy of endoscopic biopsy ranges from 50 to 100% [28]. The accuracy is low for lesions with atypical endoscopic characteristics. Endoscopic ultrasound is helpful in determining the depth of the lesions especially the extent of anal sphincter involvement and to look for perirectal lymphadenopathy. The lesions

As none of the clinical features are unique to AM, histological examination of the suspected lesions should be performed for the definitive diagnosis of AM. The main cytological features of AM are highly cellular smear, presence of binucleated or multinucleated cells, and cytoplasmic melanin pigment. However, melanin pig-

On gross examination, the lesion appears as polypoidal or ulcerated lesion with

or without brown-black pigmentation (**Figure 3**). The histological description includes cell type, degree of melanin pigmentation and mitotic index (Ki-67) [20]. Typically, AM consists of spindle-shaped (**Figure 4**) or epitheloid cells with high nuclear pleomorphism and presence of cytoplasmic melanin granules (**Figure 5**) [30]. About 20% cases are truly amelanotic on histology [31]. Four subtypes of AM based on histology are epitheloid, spindle cell, lymphoma-like and pleomorphic [32]. In the absence of melanin pigments, the tumor morphology can mimic lymphoma and gastrointestinal stromal tumors. Immunohistochemistry helps in differentiate AM from other tumors. Melanoma antigens such as S-100, HMB-45 and vimentin are positive in 78, 94 and 100% cases (**Figure 6**) [31]. The characteristic

*Gross examination of the specimen after abdominoperineal resection showing the pigmented polypoidal hard growth (arrow) of about 3 cm reaching up to the outer surface (A) and involving the anorectal junction (B).*

**54**

**Figure 3.**

*Histological examination of anorectal melanoma showing the diffuse infiltration of the tissue by spindleshaped cells with dense eosinophilic cytoplasm and pleomorphic nuclei: (A) H&E x10, (B) H&E x 20, and (C) H&E x 40. Immunohistochemical analysis revealed positive staining with c-KIT (D) and Melan A (E).*

#### **Figure 5.**

*Microscopic examination of the tumor showing diffuse infiltration of the anorectal region by large epitheloid cells with vesicular and prominent nuclei (A) H&E x 10 and (B) H&E x 40.*

#### **Figure 6.**

*Immunohistochemical analysis of the anorectal melanoma showing positivity for HMB 45 (A), Melan A (B) and SOX 10 (C).*

marker of gastrointestinal stromal tumor, c-Kit is positive in about three-fourth cases of AM (**Figure 4**) [32]. In some cases, the tumor cells may show positivity for CEA, CD30 and CD68 similar to colorectal adenocarcinoma and other tumors [33]. Hence, a panel of markers should be tested to confirm the diagnosis of AM in doubtful cases. Some unique markers for melanoma with high specificity and low sensitivity include Melanin A, Mart-1 antibodies [20]. Interestingly, Ki-67 and proliferating cell nuclear antigen (PCNA) immunostains have been found to predict survival in patients with AM [31].

Tumor-infiltrating lymphocytes (TILs) provide a reflection of the tumor microenvironment. Presence of TILs in high concentration is associated with high programmed cell death protein 1 (PD-1) [34]. High PD-1 expression indicates better prognosis for patients with AM due to good response to targeted therapies. TILs can be seen on hematoxylin and eosin stains and also with immunohistochemistry. The majority of these TILs are CD8-positive cells. In a study of 43 AM patients, TILs were present in 55% cases [35].
