Role of F18-FDG PET/CT in non-cutaneous melanomas.

Introduction: Melanoma arises from malignant transformation of melanocytes. Melanocytes are derived from neural crest cells, and migrate to skin and other non-cutaneous locations during development. Cutaneous melanoma is the most common subtype, while non-cutaneous melanomas (NCMs) account for 4-5% of all melanomas. NCMs differ in biologic behavior and have worse prognosis than cutaneous melanoma. NCMs often disseminate early by hematogenous route, and are often metastatic at the time of presentation.

Methods: Ocular melanomas are the most common type of NCMs, and uvea is the most frequent site of their origin in the eye. Mucosal melanomas can be found in mucous membranes of head and neck regions, gastrointestinal, and genitourinary tracts. Head and neck melanomas account for half of the mucosal melanomas and commonly arise in paranasal sinuses and oral cavity. Genitourinary tract mucosal melanomas are highly aggressive and have female preponderance and most commonly arise in vulva. The most common site of the urinary tract melanoma is distal urethra. Most frequent sites of gastrointestinal tract mucosal melanomas are anorectum, esophagus, and gallbladder. Anorectal melanomas usually develop from melanocytes of the squamous zone distal to the dentate line. Meningeal melanomas arise from leptomeninges, and rarely from the dura.

Results: F18-FDG PET/CT has established role in management of cutaneous melanoma, especially in the setting of lymph node and other metastases. Diagnostic difficulties maybe encountered while dealing with NCMs, especially preoperatively. Diagnosis of NCMs is established using conventional modalities, but F18-FDG PET/CT has some unique advantages in management of NCMs, which include whole-body imaging, better sensitivity for metastatic lesions, finding second primary, and guided (metabolic) biopsy. F18-FDG PET/CT is an accepted and helpful modality for staging of ocular melanoma. Liver is a common site of metastasis from NCMs and studies have shown that ultrasound, liver function tests, and chest X-ray have low sensitivity for the diagnosis of metastatic uveal melanoma, where F18-FDG PET/CT plays an important role. F18-FDG PET/CT is particularly important for assessment of such metastatic lesions and may result in avoidance of enucleation in case of ocular melanomas. F18-FDG PET/CT is also more effective than conventional imaging modalities in finding second primary, which in not uncommon in these patients. F18-FDG PET/CT may play a role in restaging before surgical salvage when localized tumor recurrence is presumed. Literature implicating the utility of F18-FDG PET/CT in NCMs is scarce and further studies are needed to validate the importance of this modality in NCMs.

Conclusions: NCMs often present in advanced stages and accurate staging is crucial for their optimal management. Better sensitivity of F18-FDG PET/CT compared to other modalities has a potential to alter the management, especially in metastatic lesions arising from NCMs.

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