Anal Melanoma: When The Uncommon Becomes A Diagnostic Challenge Case Report And Brief Review
José Luis Pérez-Guamanga,
María Paula Ordóñez-Orozco,
Mónica Alejandra Campo-Castro,
Brigith Juliana Astaiza Mondragón,
Francisco Javier Hoyos-Alvarez,
Melissa lucia Doria-Carrascal,
Daniela Stefania Herrera-Peña,
Stephanie María Torres-Ramírez,
Marlon José Rosado-Mendoza,
Maria José Vargas-Fierro,
Oscar Andres Vargas-Fierro
Anal melanoma is an uncommon neoplasm (~1 % of anal tumours) but highly aggressive, with a 5-year
overall survival below 20 %. We present the case of a 74-year-old woman who reported a rapidly growing
exophytic anal mass, pain, and haematochezia. Colonoscopy revealed a lobulated tumour 3 cm from the
anal verge; the initial biopsy showed a poorly differentiated carcinoma. Positive immunohistochemistry
for SOX10, HMB45, and Melan-A confirmed melanoma. Abdominopelvic MRI demonstrated infiltration
of the mesorectal fascia and rectovaginal septum without nodal involvement, and ¹⁸F-FDG PET/CT
ruled out distant metastases, classifying the disease as stage II (T4b N0 M0, AJCC 8th ed.). After
multidisciplinary discussion, radical proctosigmoidectomy, total hysterectomy, and abdominopelvic
lymphadenectomy with end colostomy were performed, achieving an R0 resection. Owing to the high
risk of relapse, adjuvant therapy with pembrolizumab was initiated. This case underscores the diagnostic
challenge of anal melanoma, the importance of immunohistochemistry in undifferentiated lesions, and
the value of comprehensive imaging staging to select radical surgery and/or immunotherapy. It also
highlights the necessity of including melanoma in the differential diagnosis of anal masses and adopting
an early multidisciplinary approach to optimise oncological and functional outcomes.
10.33425/2693-1516.1084
Published: 01 Nov, 2026