Primary anorectal malignant melanoma: A clinical, radiology and pathology correlation

Primary gastrointestinal melanomas are mucosal malignancies that arise from melanocytes in the oropharynx, rectum, and anus. Anorectal malignant melanoma (ARMM) are exceedingly rare, accounting for less than 1% of all melanomas, 0.1% of all rectal malignancies and 4% of anal malignancies. Diagnosis is frequently delayed as these lesions are often mistaken for haemorrhoids. Histological evaluation with special immunohistochemical stains is often necessary for definitive diagnosis. Due to the aggressive nature, 61% of patients with ARMM would already have lymph node involvement or distant metastases, by the time of diagnosis. Prognosis is usually poor with 5-year survival rate of <20%. We report a case of metastatic ARMM in an elderly lady who presented with symptoms and signs mimicking a haemorrhoid.

A 69-year-old lady presented with one year history of intermittent rectal bleed and an anorectal mass that was initially treated as haemorrhoid. Colonoscopy showed a hyperpigmented mass in the anorectal region which was confirmed as malignant melanoma on histopathological examination. Imaging with CT and MRI demonstrated locally advanced tumour with distant metastases to the liver and lung. Patient was referred for palliative management.

ARMM is a rare malignancy and often presented with non-specific clinical signs. Diagnosis is frequently delayed without high index of suspicion. MRI pelvis is the imaging of choice to assess local extent of disease. Histologic evaluation with special immunohistochemical stains is often necessary for definitive diagnosis. Prognosis is poor despite surgical and chemotherapeutic interventions.