Objective To analyze the clinical and imaging features of primary anorectal malignant melanoma (ARMM) and to improve the level of its early diagnosis and clinical management.

Methods A retrospective analysis was performed on the clinical data, multimodal imaging features, treatment methods, pathological results, and follow-up information of 31 patients with pathologically diagnosed ARMM at Zhongshan Hospital, Fudan University from December 2016 to October 2025.

Results Among the 31 patients, there were 11 males and 20 females, with age of 65 (55, 71) years. Twenty-one patients presented with hematochezia as the initial symptom (67.7%), and four asymptomatic patients were detected incidentally by physical examination. All the patients showed negative alpha-fetoprotein, carcinoembryonic antigen, and carbohydrate antigen 19-9. All 31 cases were solitary ARMM, of which 14 (45.2%) were located in the anorectal region. CT was characterized by polypoid or fungating masses, absence of intestinal obstruction, moderate to marked enhancement; MRI was characterized by specific T₁WI hyperintensity and restricted diffusion; PET/CT is characterized by high metabolism. According to Ballantyne staging, 10 cases of stage Ⅰ, 11 cases of stage Ⅱ and 10 cases of stage Ⅲ were found. Endoscopic resection was performed in 1 patient, transanal resection in 2 patients, and abdominoperineal resection (Miles operation) with routine lymph node dissection in 17 patients. The positive rates of S-100, HMB-45 and Melan-A in 20 surgical patients were 100%, 95% and 100%, respectively, with a median Ki-67 proliferation index of 65%. Median postoperative follow-up duration of surgical patients was 34.5 (2−90) months. During follow-up, 8 patients (40%) developed recurrence and/or metastasis (median time was 11 months after surgery), with the liver and lungs being the most common metastatic sites.

Conclusions Primary ARMM predominantly occurs in middle-aged and elderly women, hematochezia is the most common initial symptom. Diagnostic efficacy can be improved by combining characteristic imaging manifestations and immunohistochemical findings. ARMM is a highly malignant tumor with an overall poor prognosis, surgery remains the main therapeutic approach.