Abstract
Simple SummaryPET/CT fluorodeoxyglucose (FDG) scans are routinely used in patients to detect signs of malignant tumours or evidence of inflammation in the body. A total of 1–2% of patients show focal thyroid gland FDG uptake and 35–40% are malignant. FDG also detects metabolically active lesions containing mitochondria, known as Hürthle cells. Over 3 years, 47 patients in one hospital were found to have focal thyroid gland uptake. A total of 18 (38.2%) of the patients had malignancy, 15 (31.9%) had benign lesions that contained Hürthle cells and 14 (29.8%) had focally increased thyroid gland FDG PET/CT uptake with no cause identified. Exclusion of the Hürthle cell patients increased the risk of malignancy of the remaining PET-positive nodules from 38% to 68%. It is important to recognize Hürthle cells on FNA cytology in FDG PET/CT-positive nodules as this affects the risk of malignancy and the clinical management of focally FDG PET/CT-positive nodules.This study assesses the role of [18F] FDG PET/CT, fine needle aspiration (FNA) cytology and ultrasound in the 1–2% of patients with focally positive thyroid nodules on FDG PET/CT. All FDG PET/CT scans with focally increased thyroid FDG PET/CT uptake performed over 37 months in one institution were matched to patients undergoing thyroid FNA. Diffuse FDG PET/CT uptake patients were excluded. A total of 47 patients showed focally increased thyroid uptake. Consistent with previous studies, 18 (38.2%) patients had malignancy—12 primary thyroid carcinoma, 1 parathyroid carcinoma, 3 metastatic carcinoma to the thyroid and 2 lymphoma. A total of 15 (31.9%) lesions categorized as non-malignant contained Hürthle cells/oncocytes. A total of 14 lesions (29.8%) had focally increased FDG PET/CT uptake with no specific cytological or histopathological cause identified. No focally PET avid Hürthle cell/oncocytic lesions were found to be malignant. Exclusion of oncocytic lesions increased the calculated risk of malignancy (ROM) of focally PET avid nodules from 38% to 68%. It may be useful to exclude focally FDG PET/CT avid Hürthle cell/oncocytic lesions, typically reported as follicular neoplasm or suspicious for a follicular neoplasm, Hürthle cell type (Oncocytic) type, RCPath Thy 3F: Bethesda IV or sometimes Thy 3a: Bethesda III FNAs) from ROM calculations. Oncocytic focally PET/CT FDG avid lesions appear of comparatively lower risk of malignancy and require investigation or operation but these lesions should be readily identified by FNA cytology on diagnostic work up of focally PET avid thyroid nodules.
Highlights
Positron emission tomography using [18 F]-2-fluoro-2-deoxy-D-glucucose is diagnostically useful, as cancer cells [1] and inflammatory lesions characteristically show increased 18 F-FDG uptake due to increased rates of anaerobic glycolysis.18 F-FDG glucose is metabolized via glycolysis to 18 FDG-6-phosphate but it cannot be further metabolized intracellularly via glycolysis, so it accumulates in cells and tissues if there is increased tissue uptake. 18 F-FDG PET is commonly performed in conjunction with computerized tomography (CT)
Oncocytic focally PET/CT FDG avid lesions appear of comparatively lower risk of malignancy and require investigation or operation but these lesions should be readily identified by fine-needle aspiration (FNA) cytology on diagnostic work up of focally PET avid thyroid nodules
A total of 47 patients were identified with focally FDG PET/CT avid thyroid nodules in the 37 month period January 2017–February 2020
Summary
Positron emission tomography (known as PET) using [18 F]-2-fluoro-2-deoxy-D-glucucose ( known as 18 F-FDG) is diagnostically useful, as cancer cells [1] and inflammatory lesions characteristically show increased 18 F-FDG uptake due to increased rates of anaerobic glycolysis.18 F-FDG glucose is metabolized via glycolysis to 18 FDG-6-phosphate but it cannot be further metabolized intracellularly via glycolysis, so it accumulates in cells and tissues if there is increased tissue uptake. 18 F-FDG (referred to as FDG later throughout this article) PET is commonly performed in conjunction with computerized tomography (CT). Increased metabolic activity due to large numbers of mitochondria is present in Hürthle cell/oncocytic thyroid lesions [4,5,6] and other head and neck tumours, such as in Warthin-type salivary gland tumours [7]. FDG PET/CT avid incidental thyroid lesions 1 cm or greater in size require further investigation because of an increased risk of malignancy [8,9,10,11]. The stated risk of malignancy in FDG PET/CT focally avid thyroid nodules is approximately 35–40%, this varies quite widely depending on the case series and the patient cohort characteristics [11,13]. All focally FDG PET/CT avid nodules in our series were detected during routine investigations for either cancers at other sites, assessment of possible recurrence of thyroid carcinoma, or assessment of benign inflammatory conditions. The aim of this study was to assess the role of cytology in the risk assessment of PET/CT focally avid thyroid lesions
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