Fine-Needle Aspiration in all Thyroid Incidentalomas at 18F-FDG PET/CT

Can EU-TIRADS Revise the Dogma?

Pierpaolo Trimboli; Gaetano Paone; Giorgio Treglia; Camilla Virili; Teresa Ruberto; Luca Ceriani; Arnoldo Piccardo; Luca Giovanella

Disclosures

Clin Endocrinol. 2018;89(5):642-648. 

In This Article

Abstract and Introduction

Abstract

Objective: Focal thyroid incidentalomas (TIs) are observed in 2% of 18F-FDG PET/CT representing malignancy in one-third of cases. Currently, due to the lack of evidence on their optimal management, guidelines suggest fine-needle aspiration cytology (FNAC). The study aim was to evaluate the role of ultrasound evaluation according to EU-TIRADS to assess the risk of TIs and inform FNAC prescriptions.

Design: We retrospectively reviewed 18F-FDG PET/CT TIs recorded during the period 2014–2017. Enrolled were TIs with histological outcome and autonomous nodules. Cases with uncertain matching between 18F-FDG PET/CT, ultrasound and histology were excluded.

Results: According to the selection criteria, 75 TIs, being 13 (17.3%) malignant and 62 (82.7%) benign, were included. Cancers had significantly higher SUVmax and SUVmax ratio (Mann-Whitney P < 0.01) than benign, and the most accurate cut-offs were >7.1 and >3.65, respectively. At ultrasound, the cancer rate was 0% in EU-TIRADS 2, 2.9% in EU-TIRADS 3, 4.2% in EU-TIRADS 4% and 78.6% in EU-TIRADS 5 (chi-squared P < 0.001). Sensitivity, specificity, PPV, NPV and accuracy for malignancy were 92%, 64%, 35%, 98% and 69% for SUVmax; 85%, 68%, 36%, 96% and 71% for SUVmax ratio; and 85%, 95%, 79%, 97% and 93% for EU-TIRADS, respectively. The absence of all these three features reached a specificity of 97.1%.

Conclusions: EU-TIRADS, within a clinical careful approach, can discriminate with significant accuracy lesions at high risk of malignancy from those at low risk among TIs at 18F-FDG PET/CT. Additionally, a centre-based threshold for SUV parameters should be useful for the initial assessment of these lesions during PET/CT reading and reporting.

Introduction

In the last decades, the advent of high-resolution medical imaging tools such as ultrasonography (US), computed tomography (CT), magnetic resonance imaging (MRI) and fluorine–18 fluorodeoxyglucose (18F-FDG) positron-emission tomography/computed tomography (PET/CT) has significantly changed the management of our patients. On the other hand, the detection of unexpected asymptomatic lesions (ie incidentalomas) has been significantly increased. Indeed, due to the high frequency of thyroid disease in general population, thyroid incidentalomas (TI) have represented a frequent scenario and a relevant challenge in clinical practice.[1] In this context, the increasing worldwide diffusion of 18F-FDG PET/CT led to a significantly increased detection of incidental thyroid 18F-FDG uptake.[2–4] In particular, the overall prevalence of TI at 18F-FDG PET/CT is reported to be about 3% of all PET/CT scans,[3,4] being 1.5% for diffuse uptake and 2% focal uptake.[2–4] However, while diffuse 18F-FDG TI does not represent a warning being generally associated with autoimmune thyroid disorders (ie Hashimoto's or Graves' disease) and rarely due to thyroidal lymphoma,[3,4] focal 18F-FDG TI is of great concern as it may harbour a cancer with a rate of about one-third of the cases.[2,4] Following all the above issues, in the presence of focal TI at 18F-FDG PET/CT, the main aim is to exclude malignancy. Regardless of the above frequency of focal TI at 18F-FDG PET/CT and the not negligible cancer rate among that, there is no evidence on the optimal clinical and cost-effective management of these patients. Currently, international guidelines suggest to perform fine-needle aspiration cytology (FNAC) in all TIs at 18F-FDG PET/CT within a sonographically confirmed thyroid nodule.[5] This approach is not fully supported, being only sparse papers available for or against these recommendations and lacking data on the use of other diagnostic tools.

Ultrasound is the pivotal tool for the initial assessment of thyroid nodule. The latter is supported by a significant number of papers published during the last two decades.[6,7] More recently, several US risk stratification systems have been proposed to provide easy-to-use rules for clinical practice of thyroidologists.[5,8–11] However, whether US can be accessible, simple and inexpensive tool to identify cancers and benign lesions among TIs is a matter to be debated.[12] This study was undertaken to evaluate the role of US to assess the risk of malignancy of TIs at 18F-FDG PET/CT and inform FNAC prescriptions. Specifically, for this study, we adopted the new European stratification system for thyroid nodule, such as EU-TIRADS.[11]

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