Anorectal melanoma presenting as a polypoid lesion

A 57-year-old woman presented with intermittent painless rectal bleeding for 1 year. Colonoscopy showed a polypoid lesion 2 cm from the anus ([Fig. 1] a). Narrow-band imaging (NBI) revealed the presence of turbulent microvasculature in the surface of the lesion ([Fig. 1] b). Enhanced computed tomography revealed that the rectum was of uneven thickness and showed local enhancement; there were no enlarged lymph nodes.

Fig. 1 Colonoscopic images of a polypoid lesion 2 cm from the anus: a on white-light imaging; b on narrow-band imaging, showing the presence of turbulent microvasculature in the surface of the lesion.

In order to determine the nature of the lesion and resect it completely, endoscopic submucosal dissection was performed using a DualKnife ([Video 1]). The procedure was successful, achieving en bloc resection of the lesion. Pathologic examination revealed a malignant intraepithelial proliferation of melanocytes ([Fig. 2] a), and immunohistochemistry was positive for S100 ([Fig. 2] b) and HMB45 ([Fig. 2] c); the Ki-67 index was 60% ([Fig. 2] d). The patient was therefore diagnosed as having a malignant melanoma.

Endoscopic submucosal dissection for an anorectal melanoma that presented as a polypoid lesion.Video 1

Fig. 2 Histopathologic appearance of the resected lesion showing: a a malignant intraepithelial proliferation of melanocytes (hematoxylin and eosin [H&E] stained, ×20); b,c positivity on immunohistochemical staining for: b S100; c HMB45; d Ki-67 index of 60%.

Molecular testing revealed no BRAF, NRAS, or KIT gene mutations. Positron emission tomography–computed tomography showed no signs of tumor metastasis. Additional laparoscopic wide local excision was performed, yielding no residual tumor or lymph node metastasis. After 2 months, the patient received four cycles of adjuvant chemotherapy with temozolomide plus cisplatin. During 19 months of follow-up, she has remained well, and there has been no evidence of tumor recurrence or metastasis.

Anorectal melanoma (ARM) is an extremely rare malignancy, which accounts for only 0.5%–4.6% of all malignant anorectal neoplasms and 1.4% of all melanomas [1] [2]. ARM is usually polypoid and often melanotic [3] [4]. Amelanotic ARMs may be misdiagnosed as polyps or adenocarcinomas, which may contribute to their poor prognosis because of their highly aggressive potential. The overall median survival of ARM is reported to be 8–18.6 months [1]. This case highlights the importance of raising awareness of ARM, as having a high level of clinical suspicion could avoid misdiagnosis or delayed diagnosis, thereby improving prognosis.

Endoscopy_UCTN_Code_CCL_1AD_2AB

Conflict of Interest

The authors declare that they have no conflict of interest.

• References

• 1 Gitin A, Valdes-Rodriguez R, Canipe A. Pigmented polypoid lesion in a 24-year-old woman with hematochezia. Gastroenterology 2022; 163: e16-e17
• 2 Bordman Z, Cohen E, Hsieh E. et al. Anorectal malignant melanoma in a patient presenting with prolapsing hemorrhoids. Gastrointest Endosc 2015; 82: 417-419
• 3 Doran H. Endoscopic view of malignant melanoma of the rectum. Gastrointest Endosc 2013; 78: 785-786
• 4 Yan J, Jing J, Wu S. et al. Can transrectal ultrasonography distinguish anorectal malignant melanoma from low rectal adenocarcinoma? A retrospective paired study for ten years. BMC Gastroenterology 2022; 22: 165

Publication History

Article published online:
09 January 2024