Reply: ¹⁸F-FDG PET of Thyroid Nodules with Inconclusive Cytologic Results

REPLY: We thank Martínez-Duncker and Hurtado-López for their comments about our paper on ¹⁸F-FDG PET in thyroid nodules with inconclusive cytologic results (1). We agree with them that several other radiopharmaceuticals besides ¹⁸F-FDG PET can be considered for evaluation of solitary thyroid nodules. They refer to their recent paper on the use of ^99mTc-MIBI for this purpose (2). Like ¹⁸F-FDG PET, ^99mTc-methoxyisobutylisonitrile (MIBI) was positive in all patients with thyroid cancer (1,2). For ¹⁸F-FDG PET, this is in line with data in the literature (3,4). However, several studies in the literature do report false-negative ^99mTc-MIBI findings in thyroid cancer (5,6).

Furthermore, their study (2) was designed differently from ours (1). First, 78 of 130 patients (60%) underwent diagnostic fine-needle aspiration biopsy (FNAB). These patients do not require any additional imaging—neither ¹⁸F-FDG PET nor ^99mTc-MIBI. This type of group was not included in our study. The remaining 52 patients (40%) had inconclusive FNAB results (2). This is a relatively large percentage of patients, because inconclusive aspirates typically are reported in up to 20% of cases (7,8). The number of patients in the inconclusive-FNAB group who ultimately had thyroid cancer was also relatively high (18/52, or 35%) (2), whereas approximately 20% is typical (9). In our series, the prevalence of thyroid cancer was 14% (1).

When they separately analyzed the results of ^99mTc-MIBI imaging in the 52 patients with inconclusive FNAB results (2), 20 of 52 patients (38%) showed negative findings on ^99mTc-MIBI scintigraphy and were diagnosed with benign disease. In our study (1), ¹⁸F-FDG PET findings were negative in 25 patients (57%), all of whom had benign thyroid nodules.

In conclusion, ¹⁸F-FDG seems to be less prone to accumulation in benign thyroid nodules than is ^99mTc-MIBI, thereby allowing better selection of patients with inconclusive FNAB results who require surgical intervention. Furthermore, unlike ¹⁸F-FDG PET, the literature contains evidence that false-negative ^99mTc-MIBI findings do occur in thyroid cancer. For these reasons, we do not support the recommendation of Martínez-Duncker and Hurtado-López that ^99mTc-MIBI scans should be the first study performed in the diagnostic algorithm of patients with thyroid nodules and inconclusive cytologic results to avoid unnecessary thyroid surgery. However, a definite conclusion in favor of either ¹⁸F-FDG PET or ^99mTc-MIBI can be drawn only in a prospective comparative study with appropriately selected patients, especially in view of the high percentage of inconclusive FNAB results and the high prevalence of thyroid cancer in their series (2).