¹⁸F-FDG and ⁶⁸Ga-somatostatin analogs PET/CT in patients with Merkel cell carcinoma: a comparison study

Merkel cell carcinoma (MCC) is an aggressive neuroendocrine skin tumor. Currently, ¹⁸F-fluoro-deoxy-glucose (¹⁸F-FDG) PET/CT is the functional imaging modality of choice. Few data are available on the use of ⁶⁸Ga-somatostatin analogs. The aim of our study was to evaluate and compare the diagnostic performance of ¹⁸F-FDG and ⁶⁸Ga-somatostatin analog PET/CT in MCC patients.

Fifteen patients (12 males, 3 females; median age 73 years; range 41–81 years) with histologically proven MCC (4 with unknown primary lesion) who underwent both ¹⁸F-FDG and ⁶⁸Ga-somatostatin analog PET/CT for staging, re-staging, or treatment response assessment were retrospectively evaluated. Results of both studies were qualitatively analyzed and compared on a patient- and lesion-based analysis, using histology or clinical/radiological follow-up as reference standard for final diagnosis. According to final diagnosis, 8/15 patients had at least one MCC lesion and 7/15 had no evidence of disease. On a patient-based analysis, ¹⁸F-FDG and ⁶⁸Ga-somatostatin analogs correctly classified as positive 8/8 (100% sensitivity) patients and as negative 6/7 (85.7% specificity) and 5/7 (71.4% specificity) patients, respectively, with no significant difference. On a lesion-based analysis, ¹⁸F-FDG detected 67/75 lesions (89%) and ⁶⁸Ga-somatostatin analogs 69/75 (92%), with no significant difference. In four patients with unknown primary MCC, both tracers failed to identify the primary MCC site.

Our preliminary data suggest that ¹⁸F-FDG and ⁶⁸Ga-somatostatin analog PET/CT provide good and equivalent diagnostic performance, adding interesting insights into the complex MCC biology. However, these results do not suggest that ¹⁸F-FDG PET/CT should be replaced by ⁶⁸Ga-somatostatin receptor imaging, which should be performed in addition, according to clinical indication, to the perspective of “personalized medicine.”