Primary anorectal melanoma

Abstract

Primary malignant melanoma of the anus and rectum is a rare and aggressive neoplasm that tends to invade locally and metastasize early in the course of the disease. It is often misdiagnosed as hemorrhoids or as one of the other benign anorectal conditions and is thus linked to an overall poor prognosis and a 5-year survival rate of less than 20%. Optimal treatment is still controversial, and current evidence does not show any preferential survival benefit from abdominoperineal resection over wide local excision. Chemotherapy or radiotherapy may be used for advanced disease. We report a 71-year-old female presenting with painful bowel movements and blood in stools. She was eventually found to have a mass arising from the anorectal junction with regional lymph node involvement. The patient underwent an abdominoperineal resection and is currently scheduled for chemotherapy.

Keywords: Abdominoperineal resection; Chemotherapy; Primary anorectal melanoma.

Fig. 1

Anorectal mass, biopsy specimen: hematoxylin and eosin stain (×20) showing an atypical spindle cell proliferation with mitotic activity and without melanin. The neoplastic spindle cells have increased nuclear/cytoplasmic ratios and nucleoli.

Fig. 2

CT scan of the abdomen and pelvis showing an abnormal soft tissue mass occupying the anorectal junction, extending to the right lateral wall of the rectum.

Fig. 3

PET/CT scan showing a 1-cm metabolically active, precoccygeal lymph node.

Fig. 4

MRI of the pelvis showing a thickening of the anal wall and a mild surrounding soft tissue enhancement.

Fig. 5

APR specimen: hematoxylin and eosin stain showing spindle cells with marked nuclear pleomorphism and abnormal mitotic activity.

Fig. 6

APR specimen: immunochemical stain for HMB45 is diffusely positive, confirmatory of malignant melanoma.