A Rare Case of Hematochezia: Rectal Melanoma

Abstract

Introduction. Anorectal melanoma is an aggressive, rare subtype of both anorectal masses and melanomas. We describe a case of rectal melanoma presenting with lower gastrointestinal (GI) symptoms. Clinical Summary. A 60 year old Caucasian male with a history of Hepatitis C (no cirrhosis, on Ledipasvir/sofosbuvir) presented with 3 weeks of progressively worsening abdominal pain, constipation, hematochezia, and narrow stool caliber. On exam, the patient was hemodynamically stable and without lymphadenopathy. Rectal exam was negative for anal fissures, external hemorrhoids, or gross blood, however, noted a palpable mass in the posterior rectal wall. A colonoscopy found 6 polyps (from <5-9mm) throughout the colon that were removed with snare polypectomy, and found to be tubular adenomas. Also, a 7.2cm rectal mass involving the dentate line was visualized. Histology was positive for melanoma, and immunohistochemistry was positive for S100 and melan-A. Rectal endoscopic ultrasound showed the mass abutt the muscularis propria without clear penetration into the serosa. A staging PET scan was unremarkable. In the next 8 months, the patient underwent 4 cycles of pembrolizumab and 2 cycles of ipilimumab with radiation therapy (XRT). Discussion. Rectal melanoma (RM) is a rare cancer representing 1% of anorectal malignancies, and less than 1% of melanomas. Most literature is case series or smaller reviews. Histologically, kit and S-100 mutations are common and BRAF mutations are rare; our patient was only S-100 positive. In some metastatic case studies, kit mutations direct imantinib therapy. First line treatment is wide local surgical resection, sometimes with XRT. The role of chemotherapy is unclear. Diagnosis is difficult as symptoms are nonspecific and lesions are difficult to detect on physical exam. Even when visualized, masses are often non-melanotic. Clinical suspicion is thus essential when evaluating a patient with lower GI symptoms. Unfortunately, anorectal melanoma has a poor prognosis. Median age at diagnosis is 69, median survival is 18 months, and most patients present with at least stage II disease. Wide local excision is safer than abdominoperineal resection but does not have a relative survival advantage. Our patient was not a surgical candidate and during chemotherapy had primary tumor growth and metastases to the lung, liver, and kidney. This case highlights a rare, aggressive GI malignancy and emphasizes maintaining a high clinical suspicion.1552_A Figure 1. melanotic mass area on colonoscopy1552_B Figure 2. rectal mass on endoscopic ultrasound