Imaging Manifestations and Clinical Analysis of 12 Cases of Anorectal Malignant Melanoma

AIM: To investigate the computed tomography (CT) and magnetic resonance imaging (MRI) features and clinical characteristics of primary anorectal malignant melanoma (ARMM).METHODS: This retrospective analysis was conducted on the clinical, imaging and pathological data of 12 patients with pathologically confirmed ARMM admitted to The First Affiliated Hospital, Zhejiang University School of Medicine between March 2021 and December 2024, and the relevant literature was reviewed and summarized.RESULTS: All patients underwent CT and MRI examinations. Among the 12 patients, 5 were male and 7 were female; their ages ranged from 37 to 78 years, with a median age of 65 years. The clinical manifestations were diverse, with bloody stool being the most common (58.3%), followed by anal pain (16.7%) and polypoid vegetation prolapsing from the anus (16.7%). Typical anal pigmentation with pruritus was rare (8.3%). Gross pathological examination revealed a polypoid tumor in 7 cases and an ulcerative or infiltrative growth pattern in 5 cases. Immunohistochemical staining results showed that 12 cases were positive for human melanoma black-45 (HMB-45) and Melan-A, and 9 cases were positive for S-100. On imaging, plain CT scan showed that the tumors were predominantly isodense or slightly hyperdense, with a maximum diameter of approximately 2–5 cm. The enhancement patterns varied, but the tumors predominantly demonstrated marked heterogeneous enhancement. On MRI, most lesions were isointense on T1WI (10/12); 11/12 lesions were slightly hyperintense on T2WI, and hyperintense on diffusion-weighted images (DWI). After enhancement, most tumors exhibited marked heterogeneous enhancement.CONCLUSIONS: Most of the ARMM lesions in this group demonstrated atypical MRI signal characteristics, making diagnosis challenging. When an anorectal tumor exhibits iso- or hypointensity on T1WI and hyperintensity on T2WI, ARMM should be considered in the differential diagnosis. The CT manifestations lack specificity, and clinical symptoms may mimic those of benign lesions. ARMM may show involvement of the perirectal fat plane on imaging and regional lymph node metastasis and generally shows limited sensitivity to radiotherapy and chemotherapy, contributing to low therapeutic efficacy and poor prognosis. The definitive diagnosis of ARMM depends on histopathological examination, with immunohistochemical staining for S-100, HMB-45 and Melan-A can provide supportive information for the diagnosis of ARMM.