Unexpected post-operative diagnosis of primary rectal melanoma. A case report

Abstract

Aim: Anorectal melanoma (ARM) is a very uncommon and highly lethal malignancy. Due to its rarity and non-specific symptoms, preoperative diagnosis may be often erroneous and subsequent treatment inappropriate. We report a case of primary rectal melanoma and discuss the current diagnostic and therapeutic challenges.

Case report: An 87-year-old man was admitted to our surgical unit with a history of progressive constipation, tenesmus, rectal bleeding and transanal mucous discharge. Preoperative investigations, including CT scan and colonoscopy with biopsy, were suggestive for locally advanced low rectal sarcoma and therefore the patient underwent abdominoperineal resection (APR). However, histopathological examination and immunohistochemistry resulted in a postoperative diagnosis of primary rectal melanoma. The patient died 6 months later due to local and systemic recurrence.

Conclusion: ARM should always be considered when unusual anorectal lesions are discovered. Regardless of the pathological stage and the extent of surgery, prognosis of ARM remains poor. Thus, whenever feasible, wide local excision is now the preferred treatment, since it is associated with lower postoperative morbidity and better quality of life compared to APR. In our case, although the initial diagnosis was incorrect, APR was justified by the local invasiveness and large size of the tumor.

Fig. 1

Colonoscopy reveals a bluish-grey polypoid mass in the lower rectum, just above the dentate line.

Fig. 2

Specimen from abdominoperineal resection shows a large black ulcerated mass contiguous to the dentate line (arrow).

Fig. 3

Histopathological examination: A) The tumour was composed of epithelioid cells with diffuse growth pattern (HE × 40). B) At immunohistochemistry, neoplastic cells showed intense diffuse staining with HMB-45 (Anti-HMB-45 antibody ×40).