A case report of anorectal malignant melanoma with mucosal skipped lesion

We report our experience involving a case of relatively rare anorectal malignant melanoma with skipped lesion.

The patient was a 72-year-old man who had visited a local clinic complaining of a mass in the anal region, whereupon he was referred to our hospital on suspicion of a malignant melanoma. Close examination revealed a 25-mm black type 1 tumor one-third the size of the circumference of the anal canal and located externally to it. We performed transanal resection of the tumor and confirmed a diagnosis of malignant melanoma. Notably, multiple macular black lesions spaced away from the main lesion were observed during surgery in half of the circumference of the anal canal, from the tumor to the pectinate line. A biopsy of the area also revealed malignant melanoma; therefore, we performed abdominoperineal resection. Pathological diagnosis indicated a submucosal depth; the patient was thus diagnosed with T4 N2c M0 stage IIIb malignant melanoma and was followed on an outpatient basis.

Patients with anorectal malignant melanoma have very poor prognoses owing to early lymph node metastasis and hematogenous metastasis. Our case illustrates that small anorectal malignant melanoma lesions can spread from the main lesion and invade the mucosa; examinations may sometimes miss such skipped lesions.

Skipped lesions can occur in anorectal melanomas; thus, careful scrutiny of such lesions is required. Moreover, lesion resection is critical for anorectal malignant melanomas.