Discussion

Around 10 400 people were diagnosed with malignant melanoma in 2006 in the UK when it was the sixth most common cancer for both sexes.1 Over the last 30 years the incidence of malignant melanoma has increased more than for any other common cancer in the UK.1

Cutaneous malignant melanoma is one of the most common types of tumour to metastasise to the gastrointestinal tract;2 however, the majority of patients with gastrointestinal metastatic melanoma are asymptomatic and only up to 4.4% are diagnosed prior to death.3 Possibly due to the lack of symptoms, only a small number of patients first present with metastatic gastrointestinal malignant melanoma.

Most patients have a known cutaneous primary lesion on presentation and metastases can occur many years after the initial presentation of the primary lesion. One case study done by Tessier et al4 reviewed patients with primary and metastatic malignant melanoma from 1960 to 2000. They found 19% of patients with malignant melanoma developed metastasis and 24 patients (0.8%) were symptomatic from colonic metastases. Of these 24 patients, 5 had no history of melanoma and did not have a primary lesion at the time of presentation. Some primary sites are never found; Manouras states that the incidence of gastrointestinal melanotic metastases from an unknown primary origin ranges from 4–9% in a case series.5

Reintgen et al3 carried out a retrospective analysis of 2500 patients with malignant melanoma and found 110 (4.4%) with premortem gastrointestinal metastatic disease. However, when Das Gupta and Brasfield studied 100 autopsies of patients with known cutaneous melanoma they found 58 with small bowel metastases, 26 with stomach metastases and 22 with colonic metastases.6 Other large autopsy series have found the gastrointestinal tract to be involved in up to 60% of patients with metastatic disease.4 The inconsistency between the low incidence of clinically diagnosed gastrointestinal metastases and postmortem findings could be due to the non-specific gastrointestinal symptoms patients describe. The symptoms are similar to other types of gastrointestinal tumours—for example, abdominal pain, nausea, haematemesis, melena and weight loss.

The small intestine is the most common site for malignant melanoma metastasis and many are polypoid masses that act as leading points for intussusception, as occurred in our case. The polyps can be amelanotic (figure 1B) and may appear benign to the endoscopist. Reintgen et al found the most common sites of metastasis to be the small intestine (35%), colon (15.5%) and stomach (7%).3

Primary gastrointestinal malignant melanoma has also been described; the most common sites are the oesophagus and anorectum. Malignant melanoma accounts for up to 0.2% of primary oesophageal tumours and up to 1.25% of malignant tumours of the anus.7 Rare cases of primary malignant melanoma have also been described in the small bowel.7 Primary intestinal melanoma tends to be more aggressive and is associated with a worse prognosis than cutaneous melanoma.

Our case is of interest as the patient was symptomatic and presented with polyps infiltrated by malignant melanoma at multiple gastrointestinal sites with no demonstrable primary lesion. Literature searches found three cases regarding colonic polyps as the initial presentation of metastatic melanoma8,–,10 with no history of a primary malignant melanoma.

Sacks et al8 reported a case of a 53-year-old Caucasian woman who presented with intermittent abdominal pain for several months. Contrast barium enema revealed multiple, amelanotic, sessile polyps in the splenic and hepatic flexures and in the ascending colon. The polyps were metastatic melanoma. An upper gastrointestinal series and small bowel examination were normal. No cutaneous or primary lesion could be identified. The patient had chemotherapy but she continued to deteriorate.

Another case report by Jubelirer describes a 72-year-old man admitted with a 1-month history of mild anaemia and positive stools for occult blood.9 A colonoscopy revealed 12 sessile, amelanotic, colonic polyps. Two polyps were sent for histology and revealed metastatic melanoma. This patient stated he had two dark lesions removed from his abdomen 26 years earlier but the histological sections were not available for review. This patient died after 10 months with brain metastases.

A further case report discusses a 72-year-old man who presented with vague abdominal discomfort and weight loss over 1 month.10 Colonoscopy revealed three amelanotic polyp-like lesions in the transverse colon and histopathology confirmed these to be metastatic melanoma. No cutaneous or primary lesion was found. A CT scan showed extensive intra-abdominal lymphadenopathy and a mass in the femoral triangle. Histology revealed metastatic melanoma completely replacing lymph nodes. The patient died within 8 weeks of initial presentation.

It can be difficult to distinguish whether a patient’s disease is primary intestinal melanoma or metastasis from an unknown primary cutaneous melanoma. Blecker suggests the criteria for primary melanomas include lack of concurrent or previous removal of a melanoma or atypical melanocytic lesion from the skin, lack of other organ involvement and in situ change in the overlying or adjacent gastrointestinal epithelium.7

Researchers have suggested that gastrointestinal melanoma with an unknown primary site has developed from a regressed primary cutaneous melanoma.2 In the case we have presented the gastrointestinal melanoma was believed to be metastatic as numerous sites were affected and the patient had many melanocytic lesions, which, although none where documented as clinically malignant, are the likely source of disease.

This case illustrates that malignant melanoma can present symptomatically as gastrointestinal polyps in multiple sites with no known primary lesion. The polyps may not be pigmented and the symptoms are similar to other gastrointestinal tumours.