Comparison of ¹⁸F-FDG PET/CT and ¹¹¹In pentetreotide scan for detection of Merkel cell carcinoma

BACKGROUND: Merkel cell carcinoma (MCC) is a rare cutaneous neuroendocrine neoplasm with propensity for nodal metastases. A few reports have presented imaging results with FDG PET/CT or In- pentetreotide (OctreoScan, OCT) indicating a higher sensitivity for FDG than OCT, but no reports have directly compared FDG to OCT in the same patients. We reviewed our experience in a limited number of patients who underwent both procedures. METHODS: Patients with MCC who had FDG PET and OCT between 2000 and 2010 in our center were retrospectively reviewed. Patients were included if they had FDG PET/CT and OCT scan within a 2-month interval. For each eligible patient, we compared all abnormal lesions identified on either scan. The findings were verified by pathology or other imaging techniques up to a 4-month follow-up. RESULTS: A total of 9 patients met the selection criteria with 10 dual scans (1 was scanned twice). Three patients had no documented sites of disease at the time of imaging. One patient had negative findings on both FDG and OCT initially, but 1 year later developed FDG-positive OCT-negative disease. Five patients had metastatic disease at the time of imaging: 2 were negative on OCT and positive on FDG; the other 3 were positive on both scans but had more lesions on FDG. CONCLUSIONS: Our data on a small number of patients are in agreement with prior individual FDG or OCT reports and suggest that, overall, FDG PET/CT detects more MCC lesions and upgrades MCC stage compared with the OCT scan. Importantly, there were no lesions identified by OCT that were missed by PET. Thus, given the added resolution and sensitivity of PET, the use of OCT in MCC in the modern era is of limited value and it remains to be seen whether newer Ga-labeled somatostatin analogs will perform better than OCT.