Metastatic anorectal malignant melanoma causing ileocaecal junction obstruction due to contiguous spread

Abstract

Malignant melanoma of the anal canal is a rare and aggressive tumour associated with significant mortality. Early diagnosis and early curative surgical resection have shown to offer a survival advantage. We present a case of 53-year-old woman, who was accidentally diagnosed to have a localised lesion of malignant melanoma of the anal canal on histopathology report of the specimen of haemorrhoidectomy done for thrombosed external haemorrhoids. She refused any form of treatment and did not return for follow-up. Two years after the initial diagnosis, she presented with intestinal obstruction. The malignant melanoma had become advanced with multiple metastases to the lungs, the liver, the peritoneum and the spine. The patient underwent a diverting loop ileostomy. At the time of surgery, it was found that the primary malignant melanoma of anal canal had contiguously involved the entire large intestine up to the ileocaecal junction and hence transverse colostomy could not be done.

Background

Anorectal melanomas are rare and highly aggressive malignancies with reported incidences of 1%–3% of all anal tumours.¹ Their incidence in India has been estimated as 1.5%–2.8% of all anorectal malignancies.² After cutaneous and ocular melanomas, anorectal melanomas are the third most common type of melanomas. Risk factors for anorectal melanomas are family history and an activating mutation of CD117 (C-KIT). There are many theories attempting to explain the pathogenesis of anorectal melanomas. Oxidative stress in the local environment, immunosuppression and dysfunction of amine-precursor uptake and decarboxylation system of the gut are some of them.^{1 3 4} Anorectal melanomas have a high metastatic potential and high mortality rates. The onset of the disease is typically in the sixth decade of life with some studies reporting a higher prevalence among females.^{5 6} Anorectal melanomas usually present with clinical symptoms similar to those of other rectal disorders including bleeding, constipation, rectal pain and change in bowel habits. They are often initially misdiagnosed as haemorrhoids, thrombosed external haemorrhoids and polyps. Direct visualisation under colonoscopy with biopsy is essential in making the diagnosis.⁵

Case presentation

A 53-year-old woman presented in a surgical emergency unit with distension of the abdomen and obstipation. She had passing mucus and liquid stools since the past 1 month. She was in obstipation for the past 3 days.

She had been diagnosed as a case of malignant melanoma of anal canal, 2 years ago. The patient had presented with complaints of bleeding per rectum and was diagnosed to have thrombosed external haemorrhoids. She underwent haemorrhoidectomy. The histopathology specimen analysis revealed the presence of malignant melanoma. The diagnosis was confirmed with immunohistochemistry analysis with positivity for Human Melanoma Black 45 Antigen (HMB45) & S100. The Fluro Deoxy Glucose Positron Emission Tomography (FDG-PET) revealed that there were no distant metastases at that time. She was then advised to undergo abdominoperineal resection (APR) as it was a localised lesion. Despite multiple multidisciplinary counselling sessions, the patient refused to undergo any treatment and did not follow-up after the diagnosis.

For the complaints of persistent constipation, she had consulted a peripheral centre, where she underwent an FDG-PET scan 2 weeks back, which revealed that there were multiple distant metastases in the lungs, the liver, the spine and the peritoneum. The patient did not have any other comorbidities.

On clinical examination, the patient was vitally stable. The abdomen was distended and gross ascites was present. On local examination, there was a partially stenotic melanotic growth in the anal canal which was tender on palpation. There was no other focus of melanoma on any other cutaneous site on thorough clinical examination.

Investigations

The haemoglobin level was 81 g/L. Other laboratory investigations were normal. The X-ray of the abdomen revealed multiple air-fluid levels. FDG-PET was done 2 weeks prior at a peripheral centre, revealed multiple metastases in the lungs, the liver, the spine and the peritoneum (figure 1).

Figure 1.

Image comparing the current FDG-PET scan report (left) to the initial report (right). The multiple metabolically active distal metastases are showcased in the current scan.

Differential diagnosis

It is very important to remember that malignant melanoma might present with external thrombosed haemorrhoids. The thrombosed discoloured haemorrhoids may mask the classical pigmentation associated with malignant melanoma.

Diagnosis after the clinical and radiological investigations was clear. It was a case of primary anorectal malignant melanoma with the distant spread. It was staged to be in stage IV of the American Joint Commission on Cancer Staging System for Anorectal Melanoma.⁷

Treatment

A decision was taken to plan for decompression and diverting transverse loop colostomy as a palliative measure. The patient was counselled about the need for surgery and the changes in the lifestyle which may occur due to the stoma. The patient agreed to undergo the surgery.

After consulting the stoma nursing unit, a pre-planned site for transverse loop colostomy was marked.

Due to spinal metastasis, the patient was given general anaesthesia. A transverse skin incision was taken. On exposing the peritoneal cavity, around 1 L of ascitic serous fluid was drained. The greater omentum was studded with melanotic deposits (figure 2). The entire large colon was melanotic and frail hence could not be mobilised for a colostomy. The malignant melanoma had involved the entire large intestine up to the ileocaecal junction (figure 3). The small bowel mesentery also had melanotic deposits but the small bowel appeared to be normal (figure 4). An omental biopsy was taken and sent for histopathological analysis. Then a decision was taken to make a loop ileostomy for decompression and diversion. An intra-abdominal drain was kept.

Figure 2.

Melanotic greater omentum delivered out from the transverse skin incision made for the transverse colostomy, demonstrating the peritoneal deposits of the malignant melanoma of the anal canal.

Figure 3.

Intraoperative picture depicting the terminal ileum loops being delivered out of the surgical wound and the melanotic IC junction marked by the arrow. IC, ileocaecal.

Figure 4.

Intraoperative image with ileum loops delivered out of the surgical wound. The ileum appears to be normal and the mesentery shows melanotic deposits.

Outcome and follow-up

The patient had an uneventful recovery and the stoma became functional 3 days after the surgery. The intra-abdominal drain was removed 5 days after the surgery. After consultation with medical oncologists and radiation oncologist, palliative chemoradiation was planned.

Discussion

Malignant melanomas are composed of melanocytes, which are derived from the neural crest cells that produce melanin.^{8 9} Diagnosis of malignant melanoma of the anal canal is challenging. Most of the diagnoses are accidental being diagnosed on either colonoscopy or haemorrhoidectomies.⁵

In a case series of anorectal melanoma by Homsi et al, it was reported that 50% presented with haemorrhoids and 25% had metastasis at the time of diagnosis.¹⁰ Immunohistochemistry is a useful tool in confirming the diagnosis of malignant melanoma, in doubtful cases. S100 and HMB45 positivity on immunohistochemistry are confirmatory of malignant melanoma.¹¹ It is a highly aggressive neoplasm with approximately 32% of patients presenting with metastatic disease at the time of initial diagnosis. It spreads along submucosal planes; therefore, it is often beyond complete resection at the time of diagnosis.¹ The most common sites of metastasis include the liver, lungs, and brain.¹² In a single-institution cohort of 43 primary anorectal melanoma patients by Dodds et al, the median overall survival was 24 months.¹³ Tas¹⁴ reported a median survival of 7 months in patients with melanoma metastases to visceral sites other than the lung, and the numbers of metastatic sites were the most important prognostic factors. The 1-year survival rate for patients with a solitary metastasis was 36%, 13% for patients with two sites, and less than 1% for patients with 3 or more metastatic sites.^{14 15}

Surgical resection is the mainstay of the treatment for anorectal melanoma. There is continuing controversy over the most effective method of surgical treatment with either wide local excision (WLE) or APR.¹ There is limited research to compare the outcomes of different surgical techniques as the disease is rare. Local excision has benefits of a quicker recovery time and minimal effect on bowel function compared with APR.^{1 16} However, WLE has not proven to show a significant change in the overall outcome compared with APR. Other adjuvant therapies including chemotherapy and radiation have a less clear role in the treatment of anorectal melanoma.¹⁷ Patients with anorectal melanoma who have undergone treatment have a mean survival of 20 months.¹⁷

In our case, the primary anorectal melanoma was diagnosed at an early stage. A Curative resection was feasible at the time of diagnosis. Unfortunately, the patient did not seek any further treatment. The patient was counselled by a multidisciplinary team inclusive of psychologists. The patient was asymptomatic at that time and did not completely understand the seriousness of the disease. Within 2 years, the primary anorectal melanoma contiguously spread from the anal canal to the large intestine.

There were multiple distant metastases including metastasis to the peritoneum, resulting in peritoneal carcinomatosis. Even a simple palliative surgical procedure like colostomy was made technically challenging due to the contiguous involvement of the large intestine and hence an ileostomy was done instead. With our review of the literature, we could not find any reported case of anorectal malignant melanoma with contiguous luminal spread to the entire large intestine.

Patient’s perspective.

‘I was told multiple times by the doctors to undergo surgery when I was diagnosed to have cancer. After the surgery for haemorrhoids, my symptoms went away. I felt better. I could not believe that I had cancer. I was very uncomfortable with the idea of having the stoma surgery. The last few months have been tough. Now I have a stoma, I feel I could have taken that chance back then. I still have hope and faith that I can survive. Cancer has spread and doctors keep me informed. I am aware of my fate and I am ready to accept whatever may come my way. I feel I am responsible for my decisions and no one else. I can take some pride in that.’ The patient herself. (Translated from Marathi)

Learning points.

• Malignant melanoma of the anal canal is a rare and aggressive tumour.

• It is important to remember malignant melanoma as a possible diagnosis in the case of thrombosed external haemorrhoids. Excised specimen of external haemorrhoids should be sent for histopathological analysis.

• A high degree of suspicion and excision biopsies of the suspicious lesions at the time of colonoscopy is the cornerstone of early diagnosis of anorectal melanoma.

• Due to submucosal spread, the anorectal malignant melanoma can spread contiguously to the colon. Prompt curative surgical resection offers a survival advantage but the overall 5-year survival is very poor.

• Patient compliance and educating the patient about the aggressive nature of the tumour is very important.