Metabolic assessment of Merkel cell cancer - The role of FDG-PET/CT in rare tumors

Objectives Merkel cell cancer (MCC) is a rare, aggressive, primary neuroendocrine tumor of the skin with non-specific clinical features. The primary tumor is localized mainly in areas exposed to sun, with up to 50% in the region of the head and neck. MCC is usually diagnosed late during the course of disease. Regional LN metastases are found in up to 45% of patients at initial presentation. Distant metastases occur in liver, bone, brain, lung, skin. Following resection of the primary cancer, recurrence will occur in ~ 30% of patients. Staging was found to be the only independent prognostic factor of survival. The role of FDG-PET/CT in rare malignancies such as MCC is not well established and represents the aim of present retrospective study.

Methods Sixty FDG-PET/CT studies performed in 28 patients (M 17, age 52-99, mean 72y) were reviewed. FDG studies were performed for staging n= 18, treatment response assessment n= 16, and follow-up n=26.

Results FDG uptake was seen in 25 studies (42%) in 17 patients. 35 studies were negative. At staging PET/CT was positive in 14/18 studies. PET/CT led to change in management in 10 cases (55%) upstaging 7 patients with positive studies and downstaging 3 patients with negative studies. The group assessed for treatment response included 9/16 positive studies. PET/CT correctly diagnosed disease progression in 7, stable disease in 1 and a second primary colon cancer in 1 patient. Complete response was accurately defined in all 7 negative studies. During follow up 2/26 studies were positive. In 1 patient a positive study identified a previously unknown recurrence. The second positive study defined the whole extent of disease. None of the patients with negative studies showed any further evidence of MCC on a clinical and imaging follow up of 12-108 (median 36) months.

Conclusions FDG-PET/CT can help stage patients with MCC and define response to treatment. Based on these preliminary results FDG-PET/CT should not be used for routine follow up of patients with MCC.