The forgotten appearance of metastatic melanoma in the small bowel

Different small bowel (SB) imaging patterns have been associated with metastatic melanoma (MM). More frequently, it presents with polypoid nodules [1] causing intussusception, and less frequently as ulcerating mural nodules, exo-enteric lesions, infiltrating masses or serosal deposits. Aneurysmal dilatation is a rare presentation with only a single case previously reported in the literature to the best of our knowledge [2]. This appearance is defined as a cavitary dilatation of the intestinal lumen (> 4 cm) with a nodular, irregular luminal contour and peripheral bowel wall thickening [3] and was first described as an imaging finding characteristic of lymphoma [4]. However, it has subsequently been associated with other cancer types, including primary SB adenocarcinoma, leiomyosarcoma [5], gastrointestinal stromal tumours [6], and metastatic disease from non-small cell bronchogenic carcinoma [7], adenocarcinoma of the rectum and endometrial stromal sarcoma [8], as well as with amyloidosis [9].

Melanoma is the most aggressive form of skin cancer, with a tendency to metastasise to all organs of the human body. It is the most common metastatic tumour to the SB [10, 11]. Inevitably, most SB lesions are initially detected by CT, as the mainstream technique for staging and follow-up of melanoma patients. However, their imaging diagnosis is often challenging given the multiplicity and multitude of appearances of these lesions. Metastases to the SB, particularly in MM, can be clinically silent and can occur years after initial diagnosis and treatment [11]. However, detection of SB metastatic disease is important as patients may become symptomatic with SB obstruction, intussusception, perforation or gastrointestinal (GI) bleeding, and in some cases lead to emergency surgical intervention [12]. Additionally, early diagnosis and treatment of these lesions has been shown to improve survival and quality of life, even in palliative cases [13].

Correct and timely recognition of the different imaging patterns of SB MM can optimise and expedite patient care by guiding diagnosis, treatment and prognosis. Early diagnosis of SB metastases whilst small in size, can facilitate elective treatment and avoid non-elective hospitalisations. Here, we report the radiological and pathological findings of a series of patients found to have aneurysmal dilation of the SB due to MM, an imaging finding usually associated with lymphoma or SB primary tumours, which have different clinical management.

Gastrointestinal spread from MM is relatively common, with the SB representing the most common site of involvement [14, 15]. However, SB involvement is still vastly underappreciated clinically, with studies reporting the presence of lesions in 43.5–60% [16, 17] of cases postmortem, but only 1.5–4.4% antemortem [18]. SB melanoma metastases are frequently multiple, due to haematogenous dissemination, and preferentially affect the terminal ileum [19] and jejunum [1]. As in our reported cases, SB metastases are more common with cutaneous as opposed to non-cutaneous melanomas [10]. Primary mucosal melanomas arising in the SB are rare, remaining a controversial diagnosis as the possibility of a MM from an unidentified or regressed primary cutaneous melanoma should always be considered [20, 21].

The clinical presentation of SB MM is usually non-specific, including a constellation of symptoms associated with GI tract pathology, including abdominal pain, unexplained weight loss, iron-deficiency anaemia, change in bowel habit and GI bleeding, though a large proportion may be asymptomatic. Metastases are more frequent in the SB than primary tumours and may occur by haematogenous spread, local extension or intraperitoneal seeding [22]. Melanoma is the most common cancer type to metastasise to the SB [10, 11], occurring either at the time of initial diagnosis, or not uncommonly several years after the primary malignancy has been treated (average of 7.2 years) [11]. In our cohort, we found the SB to be the first site of MM in 2 cases, occurring on average 5 years after resection of the primary melanoma. Other primary tumours that often metastasise to the SB include breast and lung cancer [22].

The diagnosis of SB MM remains challenging due to its vague clinical presentation and limited endoscopic access. Most lesions are identified during surveillance imaging, or incidentally in patients having routine or urgent scans for other reasons. As imaging patterns of SB involvement can be diverse and mimic several other conditions, it is important to keep MM as a possible differential in patients with a prior history of melanoma, even when unusual features, like aneurysmal SB dilatation, are present. In all 3 cases, the initial radiological diagnosis was given as lymphoma, even when secondary imaging clues were present which could have helped raise concern for MM. In one of the cases, the presence of a small number of localised large necrotic lymph nodes close to the main lesion could have prompted the radiologist to consider a different diagnosis besides lymphoma, as necrotic lymph nodes are uncommon in untreated lymphoma [23]. Equally, concomitant SB lesions with diverse appearance and/or the presence of SB intussusception could have raised suspicion for MM. Finally, another tip that can guide the imaging diagnosis towards MM is the identification of other metastatic sites atypical for lymphoma, such as the adrenal glands, subcutaneous tissue and peritoneum.

Interpretation of staging CT studies of patients with MM can be challenging given the need for a thorough review of the scans and unpredictable metastatic patterns with this disease. Nevertheless, the bowel is an important review area in these patients, particularly in the case of primary cutaneous melanoma arising in the head and neck region, trunk and lower extremity [24]. On the other hand, when a SB lesion is incidentally identified on imaging, one should always consider the possibility of MM, and careful medical history should be taken regarding prior history of primary skin lesions.

In the case of a suspected SB lesion and/or in the presence of symptoms, video-capsule is still the preferred method for non-stenotic SB lesions [19]. However, CT/MR enterography/enteroclysis are valuable and more accessible techniques in the detection of small lesions. FDG-PET/CT adds the potential of detection of other secondary lesions, unknown primary or residual tumour [25‐28]. Nonetheless, definitive diagnosis of a small bowel metastasis can only be obtained histologically through surgical/endoscopic biopsy.

Modern therapeutic interventions for MM have radically expanded in the last decade and significantly improved overall survival of patients, some able to enter long term remission [29]. Although much of the survival gains are attributable to novel systemic therapies, surgical clearance of oligometastatic disease remains a key life-saving intervention for some. In the case of MM in the GI tract, surgery is key to alleviating symptoms as well as prolonging life. Therefore, early diagnosis and rapid intervention for SB MM is key to maximising quantity and quality of life, even in palliative cases [13].

Recognition of unusual radiological signs of SB spread from MM, like aneurysmal SB dilatation, is important to expedite diagnosis, provide appropriate treatment, and consequently improve quality of life and likely survival of these patients. As the most common cancer type metastasising to the SB and as a known imaging mimicker, MM should remain in any radiologist’s differential diagnosis for SB lesions. Not uncommonly, there are other imaging features which can help make the correct diagnosis.