EP286 - Surgical management of primary anorectal melanoma with regional lymphadenopathy

Purpose/Background: Anorectal melanoma is a rare and aggressive malignancy originating from melanocytes in the anorectal region. It accounts for < 2% of all melanomas and represents < 1% of lower gastrointestinal (GI) tumors. Clinical presentation is often nonspecific, with symptoms such as lower GI bleeding, anal pain, altered bowel habits, or obstruction. This frequently leads to delayed diagnosis, resulting in a high proportion of patients with regional or distant metastases on initial consult. Prognosis remains poor, with a 5-year overall survival rate of only 10-20%. Due to its rarity, standardized management protocols for primary tumors with regional lymphadenopathy but without distant metastasis are lacking.

Methods/Interventions: A 49/F presented with a 7-month history of intermittent anal pain, decreased stool caliber, and hematochezia. Physical examination revealed a bulky anterior anal mass at 2 cm from the anal verge that extends to the posterior vaginal wall, near the introitus. She also had bilateral inguinal lymphadenopathy, and no cutaneous lesions. Colonoscopy identified a mass occupying 60% of the lumen. Histopathology showed an atypical round cell neoplasm with melanin pigmentation, suggestive of melanocytic lesion, confirmed to be melanoma by HMB45 & MelanA staining.

Pelvic MRI showed a 4.96 cm polypoid mass at the lower rectum with invasion of the internal anal sphincter. An 18-FDG PET/CT scan demonstrated FDG-avid bilateral obturator, internal iliac, and inguinal nodes without evidence of distant organ metastasis. The disease was staged as primary anorectal melanoma AJCC Stage III. Following MDT discussion, taking into consideration the limited evidence supporting neoadjuvant radiotherapy or systemic chemotherapy, the team proceeded with surgical resection.

Results/Outcomes: The patient underwent an abdominoperineal resection with en bloc posterior vaginectomy, bilateral pelvic and superficial inguinal node dissection. Her postoperative course was unremarkable. Histopathology confirmed the diagnosis of melanoma with negative margins. Nodal metastases were seen in 4/19 pelvic nodes, and 4/14 inguinal nodes.

Conclusion/Discussion: Surgical resection remains the mainstay of treatment for resectable anorectal melanoma with regional nodal involvement and no distant metastasis. Further research is needed to clarify the efficacy of emerging systemic and targeted therapies, and the role of radiotherapy, in improving survival outcomes.

Figure 1. Imaging studies, intraoperative and specimen photographs, of a 49/F with anorectal melanoma. PGH, 2025
Pelvic MRI showing a polypoid mass in the lower rectum (A), with involvement of the internal anal sphincter (orange arrow) as seen on coronal view (B) and multiple enlarged obturators, internal iliac, and inguinal lymph nodes (white arrows). (C, D) Representative cuts of an 18-FDC PET/CT scan showing an FDG-avid rectal mass and enlarged lymph nodes.
Figure 1. Imaging studies, intraoperative and specimen photographs, of a 49/F with anorectal melanoma. PGH, 2025
The patient underwent an abdominoperineal resection with en bloc posterior vaginectomy. Bilateral pelvic and superficial inguinal lymph node dissection was also done. The lateral pelvic compartments after the nodal dissection are shown in (A). The inguinal nodes (B) were of the same character as the pelvic nodes-these are shown together with the resected specimen in (C).