Clinicopathologic characterization of primary anal canal mucosal melanomas: a single institution study

Primary anorectal melanomas are relatively rare, and it is recommended to staged them based on the American Joint Committee on Cancer (AJCC) melanoma staging system which relies primarily on tumor size. To date, no studies have characterized the clinicopathologic features of primary anal canal melanomas. Twenty cases of primary anal canal melanomas were included. Patients presented with a median age of 67.4 years (range: 47-88) and the male to female ratio was 1:1. Most of the patients presented with rectal bleeding (80%) and/or with a hemorrhoid-like mass (60%). The median tumor size was 2.0 cm (range: 0.8-9.7) and all were considered pT4 per the melanoma staging system whereas using the anal cancer staging system cases were considered as pT1 (9 cases, 45%), pT2 (6 cases, 30%) and pT3 (4 cases, 20%). The median depth of invasion was 9 mm (range: 1.6-3.5) and the median mitotic rate/1mm² was 11 (range: 1-31). Most tumors only extended to the submucosa (68.4%), followed by muscularis propria (15.8%) and peri-anorectal tissue (15.8%). PRAME was positive in 91% of the cases. 1 case had a BRAF D594G variant and 1 case had a KIT V560A variant. After a median follow-up of 14.9 months (range: 3.6-119), 19 patients (95%) were deceased. Only tumor extension was independently associated with disease-free survival (HR: 7.325, 95%CI: 1.316-40.779) but a trend was seen for tumor size (p = 0.078). Our data shows that for primary anal canal melanoma, the anal cancer staging system in combination with the extent of tumor invasion is more adequate for patient stratification.