An uncommon case of anorectal malignant melanoma (ARMM): Clinical presentation and surgical outcome

Anorectal mucosal melanoma (ARMM) is a rare disease with a poor prognosis. However, surgery is often difficult, due to the lentiginous growth pattern of such melanoma.

A 61 years old lady presented with anal pain for 1 year, associated with painless fresh per rectal bleeding post defecation and altered bowel habit. Physical examination showed hyperpigmentation at the anal verge, extending to the dentate line. CT, MRI and PET imaging showed localised disease. She underwent pelvic exanteration and radical lymph node dissection with gracilis flap coverage. Post operatively, she recovers well, and was discharged well on day 8. HPE came back as malignant melanoma, with 1 out 12 lymph nodes involved. She was subsequently referred to oncology, started on pembrolizumab immunotherapy.

Anorectal melanoma is an aggressive disease, often present with delayed diagnosis. Multiple imaging has been proposed, however none is standardized to diagnose ARMM. Immunohistochemical stains such as S-100 protein, MelanA and tyrosinase and with HMB-45 help in diagnosis and are sensitive for melanocytic differentiation. Surgery excision remains the most common and superior initial treatment for ARMM. One retrospective study done to compare different treatment modalities has shown that patients with surgical excision and radiation therapy had the highest median survival at 32.3 months but surgical excision remains the single best modality for ARMM.

Suspicious hyperpigmentation at the anal region should raise clinical awareness. Surgical excision with optimal margin is indicated to achieve favourable symptom control, reduce local recurrence and improve survival rate.