1. Significance of Incidental FDG Thyroid Uptake: An Educational Exhibit.
Garcia C1, Schneider M1, Van Nostrand D1, Burman K1, Wartofsky L1.
1Washington Hospital Center, Division of Nuclear Medicine
INTRODUCTION THYROID FDG PATTERNS (cont…) INCIDENTAL THYROID UPTAKE-SUV SUMMARY
Several authors report that elevated SUVs are associated
The current widespread usage of whole-body Fluorine-18 Chena et al.14 described two rare cases of diffuse 18F-
with increased likelihood of thyroid malignancy (Table The current widespread usage of whole body 18F-FDG PET
fluorodeoxyglucose (18F-FDG) positron emission FDG uptake in the thyroid that were related to 2), although this is controversial. The reasons for and 18F-FDG PET-CT studies as a screening tool for
tomography (PET) studies in the evaluation of malignant malignancy; one case harbored a papillary carcinoma discrepancy are not entirely clear, but several areas have evaluation of malignant tumors has resulted in an increase
PET
tumors has resulted in an increase in the detection of associated with Hashimoto’s thyroiditis, and the other been analyzed: 1. The higher levels of 18F-FDG uptake do in the detection of incidentally found thyroid lesions.
incidentally found thyroid lesions. Multiple reports have case was a lung cancer metastasis. Although the rate of not necessarily imply malignancy - Hürthle cell adenomas Multiple studies have demonstrated that the prevalence of
documented these lesions which may be found in up to malignancy is very low in a diffuse goiter, one must may have an average SUV >5.20; 2. partial volume effects focal thyroid lesions incidentally found on 18F-FDG PET-CT
50% of patients.1,2,3,4 When detected by 18F-FDG PET, always consider that a diffuse 18F-FDG uptake such as may underestimate SUV levels as seen in some papillary ranges from 2.2-3.7% and that when tissue is
they warrant a higher degree of suspicion than lesions secondary to chronic thyroiditis could still obscure a thyroid carcinomas; 3. TSH stimulates 18F-FDG uptake by obtained, cancer is identified from 14-47%. A small
detected by sonogram or CT alone.5,6,7 It is hypothesized thyroid nodule with focal 18F-FDG uptake.15 Focal differentiated thyroid carcinoma – increased SUV values percentage of cancers may be secondary to metastatic
CT FUSED
that FDG-avid thyroid carcinomas are less well- thyroidal 18F-FDG uptake is not likely to be misinterpreted and tumor to background ratios. The presence of any 18F- lesions to the thyroid. Diffuse thyroid uptake in most
differentiated, possess more aggressive histological as adjacent nodal uptake or physiological activity when Fig. 3: Focal thyroid 18F-FDG PET uptake. Thyroid FDG uptake on PET scan, particularly focal, seems to be instances is non-malignant. The role of SUV in determining
features, and behave more aggressively. Incidental concurrent CT images are used. (see Figure 3).16,17 Incidentaloma located in the mid-portion of the left more important in terms of clinical decision making than malignancy is variable because papillary carcinoma may
abnormalities of the thyroid gland are encountered at a thyroid lobe. the SUV level, but a histological diagnosis cannot be present with lower SUV levels and several benign thyroid
rate of 16% on cross-sectional imaging, 27% on replaced or excluded to establish the true benign or lesions, such as Hürthle cell adenomas, may present with
ultrasonography, and in 9.4% of carotid duplex scans.6,8 malignant nature of a thyroid lesion. elevated SUV. Nevertheless, the consensus amongst most
Ultrasound can detect 40% more nodules than CT, and investigators is that histological analysis of a thyroid nodule
unlike CT, can provide characterization of thyroid nodule FREQUENCY OF INCIDENTAL LESIONS with focal 18F-FDG uptake is of utmost importance to
features which may help determine the risk of determine benign versus malignant etiologies of incidental
malignancy. 9,10,11 Unlike sonography and CT imaging, Abnormal focal uptake of 18F-FDG in the thyroid on PET thyroid lesions.
18F-FDG PET provides information about tumor location, PET
or PET-CT has been reported in multiple series
biologic activity (Standard Uptake Value – SUV), and 16,17,21(see Table 1), demonstrating a low prevalence of REFERENCES
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