The Clinical Role of Dual-Time-Point 18F-FDG PET/CT in Differential Diagnosis of the Thyroid Incidentaloma

Abstract

Thyroid incidentalomas are common findings during imaging studies including 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) for cancer evaluation. Although the overall incidence of incidental thyroid uptake detected on PET imaging is low, clinical attention should be warranted owing to the high incidence of harboring primary thyroid malignancy. We retrospectively reviewed 2,368 dual-time-point 18F-FDG PET/CT cases that were undertaken for cancer evaluation from November 2007 to February 2009, to determine the clinical impact of dual-time-point imaging in the differential diagnosis of thyroid incidentalomas. Focal thyroid uptake was identified in 64 PET cases and final diagnosis was clarified with cytology/histology in a total of 27 patients with 18F-FDG-avid incidental thyroid lesion. The maximum standardized uptake value (SUVmax) of the initial image (SUV1) and SUVmax of the delayed image (SUV2) were determined, and the retention index (RI) was calculated by dividing the difference between SUV2 and SUV1 by SUV1 (i.e., RI = [SUV2 - SUV1]/SUV1 × 100). These indices were compared between patient groups that were proven to have pathologically benign or malignant thyroid lesions. There was no statistically significant difference in SUV1 between benign and malignant lesions. SUV2 and RI of the malignant lesions were significantly higher than the benign lesions. The areas under the ROC curves showed that SUV2 and RI have the ability to discriminate between benign and malignant thyroid lesions. The predictability of dual-time-point PET parameters for thyroid malignancy was assessed by ROC curve analyses. When SUV2 of 3.9 was used as cut-off threshold, malignancy on the pathology could be predicted with a sensitivity of 87.5 % and specificity of 75 %. A thyroid lesion that shows RI greater than 12.5 % could be expected to be malignant (sensitivity 88.9 %, specificity 66.3 %). All malignant lesions showed an increase in SUVmax on the delayed images compared with the initial images. But in the group of benign lesions, 37.5 % (6/16) showed a decrease or no change in SUVmax. Dual-time-point 18F-FDG PET/CT, obtaining additional images 2 h after injection, seems to be a complementary method for the differentiation between malignancy and benignity of incidental thyroid lesions.

The incidence of thyroid cancer in focal hypermetabolic thyroid lesions

“…We performed ROC analysis to determine an optimal cut-off value. Here, we found that the AUC, specificities, and sensitivities for lesion SUVmax, ratio $1\frac{TL}{TBG}$, ratio $1\frac{TL}{BP}$, and ratio $1\frac{TL}{L}$, were significantly higher than the literature values (14, 18, 20). The $1\frac{TL}{TBG}$ ratio performed best and yielded a sensitivity and specificity of 0.88 and 0.74, respectively.…”

“…The AUC was 0.82 (Table 4). Others have attempted but failed to derive SUVmax thresholds that discriminate benign from malignant lesions (14, 17, 18, 20). Many groups solely focused on differentiating benign and malignant thyroid lesions based on SUVmax.…”

Diagnostic Value of Volume-Based Fluorine-18-Fluorodeoxyglucose PET/CT Parameters for Characterizing Thyroid Incidentaloma

“…To overcome this, the Hounsfield unit ratio of thyroid nodules was assessed compared to the contralateral thyroid lobe on non-contrast CT to stratify further risk of malignancy in thyroid incidentalomas detected in 18 F-FDG PET/CT ( 27 ). Dual-time-point 18 F-FDG PET/CT and retention index were used in differential diagnosis of thyroid incidentaloma ( 28 ). Barrio et al ( 29 ) measured SUVmax, thyroid to background thyroid lesion SUVmax/thyroid background SUVmean (TL/TBG), thyroid to blood pool TL/blood pool SUVmean (BP), and thyroid to liver TL/liver SUVmean (L) ratios in benign and malignant lesions to test if intense focal 18 F-FDG thyroid uptake is associated with malignancy.…”

Prognostic significance of retention index of bone marrow on dual-phase 18F-fluorodeoxyglucose positron emission tomography/computed tomography in patients with diffuse large B-cell lymphoma

“…This was because the FDG PET image at a single time-point lacks the dynamic information regarding the FDG uptake in a lesion, and there is sometimes overlap of the standardized uptake value (SUV), that is, the semiquantitative measurement of FDG uptake, between benign and malignant tumors. The retention index (RI) of the lesion, which is defined as the percentage change in maximal SUV (SUVmax) between initial and delayed phase images, has been found to have clinical potential to predict malignance [ 12 , 13 ] and patient prognosis. [ 14 , 15 ] Another parameter derived from dual-phase FDG PET is the increment in SUVmax (SUVinc) of the tumor between initial and delayed phase image.…”