Prolapsed anorectal malignant melanoma presenting as hemorrhoids

Abstract

Rectal bleeding in a patient with a history of hemorrhoids should not be ignored. It is often benign and resolves spontaneously without treatment. Here we present a case of anorectal malignant melanoma that presented with rectal bleeding and a prolapsed rectal mass.

Anorectal malignant melanoma is a rare and aggressive disease with a poor survival rate. Delays in diagnosis are common due to clinical findings similar to those of benign anorectal diseases, such as hemorrhoids.

CASE DESCRIPTION

A 71-year-old woman with known internal hemorrhoids presented to the hospital with rectal bleeding for 3 months. She felt a “mass” inside her rectum that occasionally protruded and bled. She also complained of right upper quadrant pain and “lumps” for 3 weeks. She denied nausea, vomiting, weight loss, and changes in appetite. Her vital signs were within normal limits. Physical examination showed hepatomegaly, and rectal exam revealed an irregular mass with blood on the glove after palpation. Laboratory tests showed a hemoglobin of 9.6 g/dL; aspartate transaminase, 25 U/L; alanine transaminase, 17 U/L; alkaline phosphatase, 319 U/L (35–104 U/L); and lactate dehydrogenase, 401 U/L (135–214 U/L). Ultrasound of the abdomen showed multiple lesions in the right and left lobes of the liver, with the largest 8.3 cm in diameter. Computed tomography of the abdomen with contrast showed hepatosplenomegaly with numerous solid lesions involving the liver consistent with diffuse metastatic disease and thickening of the rectal wall.

A colonoscopy revealed a benign-appearing mass measuring 15 mm at the rectum, and a biopsy showed malignant melanoma and positive immunostaining for S-100 (Figure 1). A previous colonoscopy 5 years earlier was normal. Positron emission tomography scan confirmed metastases in the lung and liver, and the diagnosis of stage IV M1c metastatic melanoma was made. Mutation studies were negative for BRAF, V600E, and KIT. The patient was referred to a regional cancer center and received immunotherapy with nivolumab and ipilimumab. She has tolerated therapy well and has had no more rectal bleeding.

Figure 1.

(a) A large rectal protrusion measuring 15 mm. The mass appears nodular and firm. (b) Hematoxylin and eosin stain showing tumor cells arranged in sheets. Individual tumor cells showed a high nucleus to cytoplasm ratio. Both extracellular and intracellular melanin pigment was present.

DISCUSSION

Anorectal malignant melanoma is extremely rare and aggressive. Unlike cutaneous melanoma, mucosal melanomas are difficult to diagnose since they are invisible. Anorectal malignant melanoma is a rare tumor with a survival rate <10% at 5 years; the median survival rate is 24 months.^1,2 Patients who are diagnosed with anorectal malignant melanoma often have regional metastasis at the time of diagnosis.³ This tumor occurs more frequently in women. The mean ages at diagnosis are 56 years for men and 68 years for women.^4–7 Patients usually present late in their disease course due to the similarity in presentation with other common anorectal disorders, such as hemorrhoids. Symptoms include bleeding, discoloration of the skin, changes in bowel habit, anorectal pain or discomfort, and hemorrhoids because about 65% of anorectal malignant melanomas are located near the anal canal or anal verge.⁸ Approximately 30% of anorectal malignant melanomas are amelanotic, which may lead to misdiagnosis due to a benign appearance resembling benign polyps.⁹ A prompt diagnosis of these malignant melanomas is difficult since the lesion lacks pigmentation, and up to 20% of these tumors are histologically amelanotic.¹⁰

The initial management should include biopsy of the mass through colonoscopy or proctoscopy. Findings on endoscopy include a polypoid lesion in the anorectal area or an ulcerated mass with or without pigmentation; these findings can be found after hemorrhoidectomy and can be mistaken for a hemorrhoid.^8,11,12 The differential diagnosis includes Paget’s disease, Bowen’s disease, lymphoma, and gastrointestinal stromal tumor. These biopsies require careful pathologic evaluation for clinical staging, especially in amelanotic anorectal melanoma, and tumor mutation panels.¹³ KIT mutations are more prevalent in mucosal melanoma than cutaneous melanoma.¹⁴ There is no standard treatment for this condition, but previous studies have suggested treatment based on staging and include surgery (local excision and abdominoperineal resection), pelvic radiation therapy, chemotherapy, and immunotherapy.^1,6,15 With adjuvant radiation therapy followed by local excision, the 5-year rates of local control and lymph node control are 82% and 88%, respectively.¹⁶

Immunotherapy can include interferon/interleukin-2, cytotoxic T-lymphocyte antigen 4 inhibitors (i.e., ipilimumab), and programmed death 1/anti-programmed death ligand 1 inhibitors (i.e., nivolumab).¹⁷ In a pooled analysis, the response rate in patients with mucosal melanoma receiving nivolumab was 53%. Combination therapy with nivolumab and ipilimumab can improve response rates up to 60% but has more toxicity. The response rate improves in patients with PD-L1 expression >5%.¹⁸ Computed tomography and ultrasonography are important for staging and follow-up in assessing regional disease and recurrence.⁸ Common metastasis sites include the lung and liver.