Major bulk of the tumour was in the rectum with a narrow cord of tissue connecting to the anal canal. CT scan abdomen revealed metastasis to the liver and she was treated with chemotherapy

Report 
A 52-year-old widow presented with history of bleeding per rectum of 3 months duration. Blood was mostly seen mixed up with stools with occasional passage of mucus. She also had grade II hemorrhoids, which had bled 3 years back. There were no symptoms or signs to suggest gastrointestinal luminal obstruction. There were no complaints of abdominal pain or distension. She did not give any history of significant loss of body weight and her appetite was preserved.

General examination revealed marked pallor. There was no significant generalized lymph node enlargement. There was no palpable abdominal organomegaly. Digital rectal examination revealed nodularity at the upper end of anal canal extending into the rectum. She underwent flexible sigmoidoscopy, which showed a bluish-maroon proliferative mass lesion in the rectum, friable and hard with a cord like extension into the anal canal. (Fig 1)

Histopathologically rectal mucosa showed neoplastic cells with brownish black pigment (Fig 3). Round, oval and spindle shaped neoplastic cells, some of which showed nuclear pleomorphism. Melanin pigment was seen in tumor cells and macrophages. (Fig 4) 
Since CT scan of the abdomen revealed metastasis to the liver, patient was initiated on chemotherapy with dacarbazine and alpha interferon.

Discussion

Anorectal melanoma comprises less than 1% of all melanomas, and less than 1% of all anal malignancies1. The first reported case was in 1857 by Moore, who described a patient with metastatic disease(2). 

Etiology of melanoma has been linked to exposure to UV-B rays. Cutaneous melanoma is strongly linked to uv-B rays and is particularly related to sun blistering events in the childhood (3). The mechanism of this relationship is incompletely understood. Patients with Xeroderma pigmentosum, a rare autosomal recessive disorder characterized by defective repair of uv-B damaged DNA, have more than 1000 fold higher rate of skin cancer, including melanoma (4). 

One of the conundrums in melanoma etiology however is lack of site-specific association between ultraviolet radiation exposure and melanoma incidence(5). Melanoma does not regularly occur on the skin most exposed to sun such as that of the face. It also has been noted that rates of melanoma are relatively low in persons with outdoor occupations(6). Occurrence of melanoma in locations like rectum where the sun never shines is still an ambiguity. Another controversy is regarding the cell of origin; although initially thought to be extension from anal melanocytes, there is clear evidence for origin from rectal mucosal melanocytes and benign intramucosal melanocytic proliferation has been described as a marker for primary malignant melanoma of rectum (7,8) .

Anorectal melanoma seems to be more common in women and usually presents as either minimal bleeding or local mass3. As many as 80% of lesions, will be without obvious melanin pigmentation, clinically(9). Approximately 20% of lesions will be amelonotic when examined microscopically(10) This contributes to difficulty in diagnosis. The lesion is frequently ulcerated and in most series the median depth of invasion exceeds 4mm(10). These factors are known adverse prognostic indicators for primary cutaneous melanoma(11). As many as 70% of patients present with metastatic disease to regional lymph nodes or distant sites (12, 13). The lymphatic drainage from this anatomical site is complex and can involve both inguinal and mesenteric nodes. The lesion also tends to spread proximally to the rectum via the submucocoele, with an attendant risk of hematogenous spread(14). 

Surgery (Abdomino perineal resection) is the mainstay of curative treatment of anorectal melanoma because effective systemic adjuvant therapy has been lacking(15). The main determinants of prognosis are the depth of invasion and stage of the disease at presentation(16). Our patient presented with a bulky primary tumor with metastasis, hence was advised chemotherapy. Dacarbazine has been the most utilized single agent and produces response rates of approximately 20 percent usually duration of response of 3-6 months (17, 18).Responses to combination chemotherapies were equally poor (19, 20). Various combination chemotherapies and immunotherapy have also showed negative results (21, 22).A combination therapy with dacarbazine ,carmustine,cisplatin and tamoxifen was developed empirically in 1989 ,termed Dartmouth regimen (23) has produced response rates of 50 percent in small series of patients (24,25). Fotemustine and alpha interferon are newer agents used for metastatic melanoma (26, 27) of which interferon has been tried intralesionally( 28).

In conclusion, this case highlights many interesting aspects of biology of melanoma. Differences in tumor biology, particularly causation, are most likely due to differences in phenotypic expressions in various anatomical environments. Although surgery forms mainstay of treatment, multi modality approach may be required in most cases as most of them present late( 29).

References
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Figure legends

Figure 1: Sigmoidoscopic view of the tumor in the rectum; narrow cord of tumor extending into anal canal








Figure 2: Rectal mucosa showing a neoplasm with brownish black pigment H&E ´100
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Figure 3: Round and oval shaped neoplastic cells with some showing nuclear pleomorphism. Melanin pigment is seen in tumor cells and macrophages. H&E ´400
[CLICK TO ENLARGE]

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