The transition from stable to progressive disease is unpredictable in patients with biochemical evidence of medullary thyroid carcinoma (MTC). Calcitonin and carcinoembryonic antigen (CEA) doubling times are currently the most reliable markers for progression, but for accurate determination, serial measurements, which need time, are required. We compared (18)F-FDG PET and (18)F-dihydroxyphenylanaline ((18)F-DOPA) PET with biochemical parameters and survival to assess whether these imaging modalities could be of value in detecting progressive disease.
Methods: We evaluated the outcome of (18)F-FDG PET or (18)F-DOPA PET with calcitonin and CEA doubling times in 47 MTC patients. A subgroup of patients was included in the whole metabolic burden (WBMTB) analysis, with determination of standardized uptake values and number of lesions. WBMTB of (18)F-DOPA PET and (18)F-FDG PET was compared with biochemical parameters. Furthermore, survival was compared with (18)F-DOPA PET or (18)F-FDG PET positivity.
Results: Doubling times were available for 38 of 40 patients undergoing (18)F-FDG PET. There was a significant correlation with (18)F-FDG PET positivity. Doubling times were less than 24 mo in 77% (n = 10/13) of (18)F-FDG PET-positive patients, whereas 88% (n = 22/25) of (18)F-FDG PET-negative patients had doubling times greater than 24 mo (P < 0.001). Between doubling times and (18)F-DOPA PET positivity, no significant correlation existed. (18)F-DOPA PET detected significantly more lesions (75%, 56/75) than did (18)F-FDG PET (47%, 35/75) in the 21 patients included in WBMTB analysis (P = 0.009). Calcitonin and CEA levels correlated significantly with WBMTB on (18)F-DOPA PET, but doubling times did not. (18)F-FDG PET positivity was a more important indicator for poor survival in patients for whom both scans were obtained.
Conclusion: (18)F-FDG PET is superior in detecting patients with biochemical progressive disease and identifying patients with poor survival. Although (18)F-DOPA PET has less prognostic value, it can more accurately assess the extent of the disease in patients with residual MTC. Hence, both scans are informative about tumor localization and behavior. On the basis of these results, we designed a clinical flow diagram for general practice in detecting recurrent MTC.