Rectal melanoma--a rare tumour

Malignant rectal melanoma is a rare tumour. We report a case of a 66-year-old man who presented with a two-month history of rectal bleeding, pain, and tenesmus. A semicircular rectal tumour was seen, just above the dentate line. Biopsies proved it to be an amelanotic malignant melanoma, as protein S100, melanoma antigen HMB45 and Melan-A expression were found. CT scan and rectal ultrasound showed invasion into the internal sphincter and several enlarged perirectal nodes. No distant lesions were detected on CT scan, nor on PET scan. An abdominoperineal resection was performed as a substantial part of the internal anal sphincter was invaded. Histology confirmed an amelanotic malignant melanoma. The patient recovered well from the operation, and received no adjuvant therapy. Four months later, multiple liver metastases were seen on CT scan. With this case we want to illustrate that malignant rectal melanoma can be difficult to diagnose, as patients have non-specific symptoms, and histology may be misleading. One should always check for protein S-100, melanoma antigen HMN-45 and Melan-A expression, as they are strongly suggestive of melanoma. Wide local excision is the preferred procedure when technically feasible, but abdominoperineal resection has to be done if the tumour invades a substantial portion of the anal sphincter or is circumferential. Rectal melanoma has a poor outcome with a 5-year survival rate of between 10-20%. The extent of the disease correlates with the overall survival. The role of radiotherapy, chemotherapy or immunotherapy looks promising, but further investigations are needed.