Diagnostic Accuracy of 18F-FDG PET in Restaging Patients with Medullary Thyroid Carcinoma and Elevated Calcitonin Levels

Medullary thyroid carcinoma (MTC) is a rare endocrine tumor arising from the C-cells of the thyroid gland. Calcitonin is the principal serum tumor marker. A rising calcitonin level after total thyroidectomy for localized disease generally indicates residual, recurrent, or metastatic disease. The role of 18F-FDG PET in MTC remains somewhat unclear. We reviewed our own experience with 18F-FDG PET in postthyroidectomy MTC patients with elevated calcitonin. Methods: From our database, we identified patients with suspected residual, recurrent, or metastatic MTC and elevated calcitonin who had been referred for 18F-FDG PET between January 2000 and October 2005. 18F-FDG PET findings were classified as positive or negative on the basis of visual interpretation of the scan. Standardized uptake values (SUVs) were also calculated. The 18F-FDG PET findings were verified by histopathologic examination, when available, or other imaging studies and clinical follow-up. Any negative 18F-FDG PET result was considered false-negative. Results: Twenty-eight patients underwent a total of 38 18F-FDG PET studies. Calcitonin levels ranged from 106 to 541,000 pg/mL (median, 7,260 pg/mL). There were 23 true-positive, 1 false-positive, and 14 false-negative 18F-FDG PET scans, yielding an overall sensitivity of 62%. There was no true-positive finding when calcitonin levels were below 509 pg/mL (n = 5). Using an arbitrary cutoff of 1,000 pg/mL, we found that the sensitivity in scans with calcitonin levels greater than 1,000 pg/mL increased to 78% (21/27; 95% confidence interval, 58%–91%). The mean SUV of all lesions with 18F-FDG uptake was 5.3 ± 3.2 (range, 2.0–15.9). Among the 14 patients with false-negative 18F-FDG PET findings, 8 had concurrent anatomic imaging studies and only 2 of these had positive findings. Conclusion: 18F-FDG PET can detect residual, recurrent, or metastatic MTC with a reasonable sensitivity of 78% when the calcitonin level is above 1,000 pg/mL but appears of limited use if the calcitonin level is below 500 pg/mL.