Malignant rectal melanoma – a rare case study and literature review

Summary

Primary malignant rectal melanoma is an extremely rare disease manifested by rectal bleeding. The diagnosis is established by endoscopy and confirmed by histology. Unfortunately, the disease is often diagnosed in the advanced stage. Treatment is mainly surgical. However, the intervention scope is still under discussion. Given the low occurrence of cases, optimal management of this disease is ambiguous. In this paper we describe the case of a 55-year-old woman with a malignant rectal melanoma and also present the literature review of these issues.

Key words:
malignant melanoma – rectum – cancer – abdominal-perineal resection – surgery

Introduction

Malignant melanoma is a malignant tumour of the pigment cells, melanocytes. The usual location is the skin, but it can also occur in the mucous membranes or in the eye. Malignant change in the structure and propagation of pigment cells causes melanoma. Malignant rectal melanoma was first described by Moore in 1857 [1,2]. Primary malignant rectal melanoma is an extremely rare malignant disease manifested mainly by rectal bleeding, tenesmus, or the sensation of a foreign body in the rectum. Primary rectal melanoma accounts for 1% of all rectal malignancies. The rectum is the third most common location of malignant melanoma occurrence. The highest prevalence of the disease is in women aged 50–60 years [3–5]. In contrast to other forms, malignant rectal melanoma lacks the association with exposure to ultraviolet light. The diagnosis is specified by histology. Due to the occurence of only a few cases, the optimal management of this disease is ambiguous. Treatment is mainly surgical, but the extent of surgical treatment is constantly under discussion. Radiotherapy and chemotherapy used for patients with malignant rectal melanoma are considered to be only palliative. The prognosis is poor due to the fact that the disease is usually diagnosed at an advanced stage. The median survival for pa­tients with malignant rectal melanoma is 24 months with five-year survival of less than 10% [6].

Case study

In our case we have a 55-year-old woman who visited a doctor because she had fresh blood stains in the stool, without any other difficulties, for a month. Per rectum examination: 2 cm elastic tumour growing into the lumen about 2 cm from the anocutaneous line. Colonoscopic examination: grade I haemorrhoidal knots, 2 cm pink smooth polypoid tumour in the anal canal; the rest of the colon – normal finding. A histology specimen was taken during the colonoscopy with the conclusion of amelanotic epithelioid high-risk malignant melanoma. A CT scan revealed pulmonary parenchyma without focal changes; a small hypodense 4 mm nodule in the 8^th segment of the liver under the dome of the diaphragm; the retroperitoneum and mesentery without pathologically enlarged lymph nodes (fig. 1); the uterine body bearing a 27 × 24 mm susp. myomatous uterus; a hypodense formation of 24 × 12 mm just above the internal anal canal; around the right parailiac vessels two lymph nodes up to 1 cm, otherwise no other lymphadenopathy (fig. 2). The histology was reviewed by another pathologist with the conclusion of a mucosal malignant melanoma. This was consulted with the oncologist and the surgeon who indicated radical surgery. The patient underwent an abdominoperineal resection and lymphadenectomy with construction of terminal descendent ostomy, hysterectomy with bilateral adnexectomy for uterus myomatosus and resection of the focus in the 8^th segment of the liver (fig. 3). Intraoperative ultrasonography was without any other focal changes in the liver. The postoperative healing process was without complications and on the 7th postoperative day the patient was released to outpatient care. The histological findings confirmed the 28 mm nodular amelanotic melanoma in the anal canal, with metastatic melanoma in all lymph nodes, no apparent perinodal promotion and a 4mm metastasis of a malignant melanoma in the liver (fig. 4, 5). A PET scan was performed without findings of the active disease and the patient was recommended radiotherapy of the perineal, pelvic and inguinal areas.

Discussion

Primary anorectal melanoma is a very rare disease with high malignant potential and a very poor prognosis. There are only few references to the disease, mostly as case reports, or as small groups of patients, in literature [1,2]. From 1939 to 1996, 428 patients with primary rectal malignant melanoma [6] have been documented around the world. Primary anorectal melanoma is a disease representing about 1% of all anorectal malignancies. The rectum is the third most common site of malignant melanoma after the incidence of the skin and eye, which represents about 0.2%–3% of all malignant melanomas [5,7]. The disease occurs mostly after forty years of age with the highest incidence between the fifth and the sixth decade, and is predominant in women.

The most common clinical symp­toms include: rectal bleeding, tenesmus and the sensation of a foreign body in the rectum. Endoscopy and computed tomography have the most relevant results for diagnosis and staging of the disease [8]. Diagnosis is confirmed by histological and immunohistochemical evidence of melanin, S-100 protein and HMB-45.

The prognosis is poor: five-year survival is given below 5%, and at the time of diagnosis 60% of patients have distant metastasis or lymph node metastasis [9]. Factors involved in the poor prognosis of the disease are: ulceration of the tumour, the biological nature of the tumour, high congestion of rectal mucosa, and hematogenous metastasis [10,11]. Synchronous presence of visceral metastases and pelvic nodal involvement are the results of high metastatic potential.

The only potentially curative modality is surgical resection. The aim of the surgical treatment is to achieve local control and prevention of local recurrence of the disease. There is no consensus as to the extent of surgery. Several studies have considered the method of choice to be abdominoperineal resection with lymphadenectomy [9,12,13]. The principle of this procedure is based on the assumption that the disease has spread beyond the submucosa proximal to the mesenteric lymph nodes. Other studies suggest only restorative sphincter surgery in terms of the wide excision, either in combination with adjuvant radiotherapy to the tumour area and regional pericolic and inguinal lymph nodes, or not [13–15]. Radiotherapy and chemotherapy alone are considered to be only palliative treatment of locally advanced or metastatic colorectal malignant melanoma [15].

Conclusion

Malignant rectal melanoma is a rare disease. Prompt diagnosis and adequate treatment based on a multidisciplinary approach is essential for the prognosis of the patient. Unfortunately it is a disease with high malignant potential, and overall survival of pa­tients with malignant rectal melanoma is low.

The author declares he has no potential conflicts of interest concerning drugs, products, or services used in the study.

The Editorial Board declares that the manuscript met the ICMJE „uniform requirements“ for biomedical papers.

Submitted: 20. 1. 2014

Accepted: 8. 3. 2014

Robert Duchoň, MD

Department of Surgical Oncology

Slovak Medical University and National Cancer Institute

Klenova 1, 833 10 Bratislava

Slovak Republic

robert.duchon@nou.sk