Primary amelanotic melanoma of the rectum mimicking adenocarcinoma

PATIENT Male, 55 FINAL DIAGNOSIS: Melanoma Symptoms: Worsening constipation • tenesmus • weight loss MEDICATION - Clinical Procedure: Chemoradiation therapy Specialty: Oncology. OBJECTIVE Challenging differential diagnosis. BACKGROUND Malignant melanoma is usually readily diagnosed by the presence of melanin granules. Although amelanotic melanoma contains a few melanin granules, it is often difficult to differentiate from non-epithelial malignant tumors. Immunohistochemical staining may be needed to diagnose the condition. CASE REPORT This report describes a case of amelanotic melanoma of the rectum, which was originally suspected to be an adenocarcinoma, but was subsequently correctly diagnosed by immunohistochemical staining with HMB-45 antibody and by the presence of S-100 protein. A pinkish-red ulceroproliferative growth was located about 7 cm from the anal verge. The patient was treated by laparoscopic low anterior resection. CONCLUSIONS Very few cases of amelanotic melanoma of rectum have been reported in the literature and there is only limited clinical experience with this disease. It appears to be a highly lethal tumor and may need much more aggressive treatment than that used for carcinoma of the rectum.