Melanoma Mimicking a Polyp in an Unusual Location- A Rare Case Report

Dr. Yamini P M¹, Dr. Akhilesh. R*², Dr. Sarah Grace³, Dr. Vimal Chander⁴, Dr. Sonti Sulochana⁵, Dr. Venkat Raghavan ATM⁶

1,2,3,4,5,6. Department of Pathology, Saveetha medical college, Chennai, Tamilnadu, India.

*Correspondence to: Dr. Akhilesh. R, Department of Pathology, Saveetha medical college, Chennai, Tamilnadu, India

Copyright

© 2023 Dr. Akhilesh. R. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Received: 08 June 2023

Published: 01 July 2023

DOI:10.5281/zenodo.10029615

Abstract

Anorectal melanomas are rare and have a far worse prognosis as compared to cutaneous melanomas. They can be highly deceptive with their presentation like bleeding and mass descending per rectum, that can often mimick some benign lesions such as hemorrhoids, polyps and also ano-rectal carcinomas unless a histological diagnosis of melanoma is made. Here, we present a case of anorectal melanoma in a 75-year old male who presented with bleeding per rectum for 2 months, following which an abdomino-perineal resection was performed based on a preliminary diagnosis of rectal malignancy. However histopathological examination revealed an anorectal melanoma, which was further confirmed with the help of immunohistochemistry.

Key words: anorectal melanoma, polyp, prognosis

Introduction

Melanomas most commonly arise from skin, but can also be found in retina and other non-cutaneous mucosal sites like vulvo-vaginal region, head and neck region, urinary tract, etc with anorectum being the third most common site of mucosal melanomas.(1) Mucosal melanomas constitute 1.3% of all melanomas, which is incredibly rare(2) and only 0.05% of colorectal cancers are anorectal melanomas (AM).(3) Melanomas are more prevalent amongst women in their fifth and sixth decades respectively.(1) The muco-cutaneous junction is where the majority of cases take origin.(4) Patients typically present with altered bowel habits, anal masses, anorectal discomfort, or bleeding per rectum. The annual incidence of anorectal melanoma in Americans from 1973 to 2011 was studied using the SEER database which showed 0.259 cases per 100,000 men and 0.407 cases per 100,000 women, this has progressively risen over time.(5,6) Melanomas are rare and aggressive tumors with poor prognosis. Surgical resection along with chemotherapy and radiotherapy remains the mainstay of treatment.

Case History

A 75 year old male presented with complaints of bleeding per rectum for 2 months which on further examination revealed a growth in the anorectal junction. Robotic assisted abdominoperineal resection (APR) was performed and the specimen was sent for histopathological examination. Grossly, the specimen appeared intact and measured 28cm in length which included parts of sigmoid colon, rectum and anal canal. Cutting open the bowel wall revealed a grey-white to focally black polypoidal growth measuring 4.5x4x4cm involving the anorectal junction extending into middle and lower third of the rectum. Cut surface of the polypoidal mass appeared tan-white, firm and focally black. (Fig.1a,b) On histology, sections showed polypoidal anorectal junctional mucosa (Fig.2a) showing a malignant neoplasm arranged in sheets and nests of polygonal cells (Fig.2b) with moderately pleomorphic, vesicular nuclei with prominent nucleoli (Fig.2c) and dark brown intracytoplasmic melanin pigment deposition. (Fig.3a) Mitotic activity was brisk (14-16/10HPF). (Fig.2d) Further examination showed the tumor infiltrating the mucosa and extending into muscularis propria. There was no lymphovascular or perineural invasion identified. Few multinucleated tumor giant cells were also identified. (Fig.3b) There was no areas of necrosis noted and all the regional lymph nodes (5 lumph nodes) examined were free of tumor. On immunohistochemistry, the tumor cells showed diffuse cytoplasmic positivity for HMB-45, (Fig.3c,d) thereby confirming the diagnosis of melanoma.

Discussion

Anorectal melanoma (AM) was first reported by Moore in 1857 and is an extremely rare malignant tumour arising from anorectal melanocytes. (7) Grossly, the tumours can be polypoidal with or without pigmentation. Histologically, four cell types have been reported which include epitheloid, spindle cell type, lymphoma-like and pleomorphic respectively. (1) AMs showed a higher incidence of KIT mutations in a study conducted by Priyadarshini et al(8) which was concordant with that of prior reports. BRAF mutation may be detected in about 50% patient with cutaneous melanomas, however they are identified in only 4.5–15% patients with anorectal melanoma. (2)

AMs appear to be melanotic in about 30% of case. (9) Amelanotic AM is frequently mistaken for other anorectal diseases, such as polyps, haemorrhoids, and anorectal cancer.  The differential diagnosis for amelanotic melanomas microscopically can be lymphoma, undifferentiated carcinoma, poorly differentated adenocarcinomas and squamous cell carcinoma, respectively. Immunohistochemical markers that can help confirm the diagnosis are S-100, Melan-A, Ki-67 and HMB-45. Amelanotic melanoma in AMs has been linked to worse prognosis in several studies as compared to melanotic melanoma.(10) They are more likely to be invasive, misdiagnosed and have significantly poor five-year survival rates that is estimated to be fewer than 20% (11)

Pathological staging of primary anorectal mucosal melanoma is often performed according to the American Joint Commission on Cancer (AJCC) guidelines for cutaneous melanoma, as an anorectal melanoma-specific staging system does not exist till date.(8) Studies were conducted by classifying AMs into 3 stages as follows: stage I is local disease, stage II is regional disease and stage III is disseminated disease. Majority of patients had either regional (stage II) or disseminated disease (stage III) at the time of initial diagnosis according to Vikas et al(2) which coincided with the previous studies conducted from India.(12,13) This may be due to delayed presentation at healthcare facilities due to non-specific symptoms, amelanotic picture on histology or sometimes a misdiagnosis.(11) The common site of metastasis includes liver followed by lung.(2) Treatment modalities include surgery (APR or wide local excision) followed by chemotherapy and immunotherapy. Patients with metastatic disease require palliative treatment.

Conclusion

Anorectal melanomas are rare and usually have a poor prognosis. The tricky clinical presentations, variety of histological findings and inaccurate pathological reporting can all contribute to a delayed diagnosis or even a misdiagnosis. Hence, clinicians and pathologists must be aware of its varied clinical and morphological spectrum and the diagnostic difficulties associated with it to prevent its misdiagnosis. Also extensive tissue sampling along with immunohistochemical markers can help in a long way to establish the diagnosis.

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