Clinical added-value of 18FDG PET in neuroendocrine-merkel cell carcinoma.

Merkel cell carcinoma (MCC) is a rare and highly malignant skin cancer with neuroendocrine differentiation. We studied the potential value of 18FDG PET in the management of MCC. Eleven patients with MCC were examined by 18FDG PET and PET-CT for staging purpose (n=4) or for detection of recurrence (n=7). Qualitative and quantitative interpretation of PET studies was performed routinely. 18FDG PET observations were compared to clinical and radiological findings. In 6 patients, PET findings were also compared to histology. In 7 patients, the 18FDG tumor uptake was compared to the MCC proliferative activity expressed by the Ki-67 index. 18FDG PET was contributive in 10/11 MCC patients. In 7 patients, 18FDG PET detected focal lesions or a disseminated stage of the disease including dermal, nodal and visceral metastases. In 3 patients, a normal 18FDG PET confirmed complete remission of disease. Most MCC patients exhibited highly 18FDG-avid sites suggestive of increased glucose metabolism. This imaging pattern was related to a high proliferative activity (Ki-67 index >50%). In 1 patient with a weakly proliferative nodal MCC (Ki-67<10%), a false negative result was yielded by metabolic imaging. In 4/11 patients, 18FDG PET revealed an unsuspected second neoplasm in addition to MCC. It is concluded that whole-body 18FDG PET may be useful in the management of MCC patients. However, a normal 18FDG PET aspect cannot rule out MCC with low proliferative activity.