FDG PET/CT outperforms FLT for thyroid cancer imaging

A comparison between glucose mimicking F-18 FDG and F-18 FLT, which tracks cell proliferation, pegged F-18 FDG PET/CT as a better imaging technique for the diagnosis of differentiated thyroid metastases, according to a study published in the April issue of Radiology.

Masatoyo Nakajo, MD, PhD, from the department of radiology of Kagoshima University Graduate School of Medical and Dental Sciences, Sakuragaoka, Kagoshima, Japan, and colleagues completed a comparative study of the two radiotracers and the results were clear.

“FDG PET/CT is superior to FLT PET/CT in the diagnosis of postoperative differentiated thyroid cancer lymph node and dis­tant metastases,” wrote the authors. “Thus, FDG PET/CT is more suitable than FLT PET/CT for examining recurrence of postoperative differenti­ated thyroid cancer.”

Primary differentiated thyroid cancers develop in either the papillary or epithelial cells and metastasize in approximately 10 to 15 percent of patients. Ordinarily thyroid cancer has a good prognosis, but patients do not usually fare well with malignancies of this kind. The study noted that about 40 percent of patients with metastatic differentiated thyroid cancer live past the 10-year mark following diagnosis of metastases.

The F-18 based imaging agents capture areas of hypermetabolism or cell proliferation, which prove useful in the detection of metastases, but researchers compared them one on one to see which had the most diagnostic value. They concluded that F-18 FDG was more sensitive and more specific for the detection of metastatic recurring differentiated thyroid cancers in post-operative patients.

Results of this prospective study which included 20 patients with differentiated thyroid disease showed that F-18 FDG was found to be 92 percent sensitive and 86 percent specific on a per-patient basis in the detection of metastatic differentiated thyroid cancer. F-18 FLT delivered a sensitivity of 69 percent and a specificity of 29 percent. 

FDG was 85 percent sensitive and 99.5 percent specific in the detection of lymph node metastases, whereas F-18 FLT sensitivity was 50 percent and specificity was found to be 90.7 percent. False positives for nodal metastases were higher for F-18 FLT than for FDG PET studies.

F-18 FDG appeared to be limited in its sensitivity, 40 percent, to distant metastases but far exceeded FLT, which delivered a 6.8 percent sensitivity. Researchers attributed this to lower resolution of PET when imaging small lung metastases.

Researchers also compared uptake of these two tracers with whole body I-131 scans to find discordant areas of tracer distribution.

“There are radioiodine-positive and radioiodine-negative metastases from postoperative differentiated thyroid cancer,” explained the authors. “The former can be detected and treated with I-131 under TSH stimulation. Radioiodine-negative metastases neces­sitate other therapies, including sur­gery, external radiation therapy, che­motherapy, and/or careful surveillance. Therefore, it is crucial to find them for patient treatment.”

A total of 73 distant metastases and 34 nodal metastases were detected. A combination of I-131 and F-18 FDG provided the highest sensitivity, 97 percent, for nodal metastases. About 34 percent of distant metastases showed positive uptake only with F-18 FDG.

“It has been reported that FDG accu­mulates mainly in differentiated thyroid cancer metastases that are negative for I-131 uptake and that the difference in uptake between them may reflect vary­ing cancer cell differentiation.”

Further studies are needed to validate F-18 FDG for potentially better characterization of malignancies, especially nodal metastases, in post-therapeutic differentiated thyroid cancer patients.