Anal melanoma
Anorectal melanoma; mucosal anal melanoma
Anal melanoma is a rare and aggressive mucosal melanoma — approximately 1% of anal cancers and 1–2% of all melanomas. It typically presents late as bleeding, mass or change in bowel habit (often initially diagnosed as a thrombosed haemorrhoid). Median age 60–70; slight female predominance. Like other mucosal melanomas, BRAF mutations are uncommon while KIT and NRAS mutations are over-represented. Surgical management has shifted from radical abdominoperineal resection to wide local excision wherever feasible — local control is comparable while preserving sphincter function and quality of life. Five-year overall survival remains 10–20% reflecting both biology and late presentation.
Clinical features
- Anorectal bleeding, palpable anal mass, change in bowel habit, anal pain or pruritus, faecal incontinence.
- Often initially misdiagnosed as thrombosed haemorrhoid or anal polyp until biopsy.
- Pigmented (most cases) or amelanotic (~ 20%) ulcer / mass in the anal canal; rectal extension common.
- Inguinal and mesorectal lymphadenopathy at presentation in many.
- Median age 60–70; female predominance slight.
Diagnosis
- EUA with biopsy is essential — any suspicious anal lesion warrants biopsy; HMB-45 / Melan-A / S100 / SOX10 IHC.
- Imaging — MRI pelvis (depth, sphincter, mesorectal nodes), CT NCAP, brain MRI, FDG-PET.
- Molecular — BRAF V600 (rare), KIT (exons 11, 13, 17), NRAS Q61, occasionally GNAQ / GNA11.
- Multidisciplinary review with colorectal surgery, oncology, melanoma MDT.
Staging
- First distinguish true anal canal / anorectal mucosal melanoma from cutaneous melanoma of the perianal skin; perianal skin lesions follow the cutaneous melanoma pathway.
- For true anorectal mucosal melanoma there is no single universally accepted validated AJCC mucosal TNM equivalent to cutaneous melanoma. Document local extent on MRI, sphincter / rectal involvement, mesorectal and inguinal nodal basins, and distant disease.
- Ballantyne system (local / regional / metastatic) is historically used and remains a simple clinical descriptor alongside modern imaging-based MDT staging.
Management
- Localised disease — wide local excision (WLE) preferred over abdominoperineal resection (APR) where achievable. Multiple series show comparable local-recurrence and survival with sphincter preservation.
- APR reserved for: tumours involving the upper anal canal / lower rectum, bulky disease where WLE margin not achievable, sphincter destruction.
- Lymph-node management — SLNB technique not standardised in anal melanoma; therapeutic inguinofemoral or mesorectal dissection for clinically positive nodes.
- Adjuvant therapy — anti-PD-1 (nivolumab, pembrolizumab) at MDT for resected high-risk disease; combination ipi + nivo for higher-risk.
- Advanced / metastatic — first-line ICI; KIT-mutant disease may respond to imatinib; BRAF-mutant disease (uncommon) — D+T or E+B.
- Adjuvant RT — for positive margins or extensive locoregional disease; improves local control.
Prognosis and surveillance
- 5-year overall survival 10–20% historically; improvements with modern systemic therapy.
- Adverse features — tumour thickness, ulceration, perineural invasion, lymphovascular invasion, positive nodes, T4 status.
- Surveillance — 3-monthly clinical examination and imaging for 2 years; then 6-monthly to 5 years.
- Late recurrence (years) is well-described — lifelong surveillance.
References
- Pessaux P et al. Anorectal melanoma — wide local excision vs abdominoperineal resection. Dis Colon Rectum; 2004.
- Iddings DM et al. Practical management of anorectal melanoma. Ann Surg Oncol; 2010.
- Mole RJ et al. Anal melanoma — current evidence and management. Colorectal Dis; 2014.
- Amin MB, Edge SB, Greene FL et al., eds. AJCC Cancer Staging Manual. 8th ed. New York: Springer; 2017. See mucosal melanoma staging.
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