From the Guidelines
Hypermetabolic thyroid nodules on PET scans should be thoroughly evaluated with a dedicated thyroid ultrasound and fine-needle aspiration (FNA) biopsy due to the high risk of malignancy, which ranges from 30-50%. When a thyroid nodule shows increased FDG uptake on a PET scan, as seen in 80-90% of differentiated thyroid carcinoma cases 1, a comprehensive evaluation is necessary to determine the presence of malignancy. The evaluation should include a dedicated thyroid ultrasound to assess features suggesting malignancy, such as irregular margins, hypoechogenicity, microcalcifications, taller-than-wide shape, and increased vascularity.
- Key features to evaluate on ultrasound include:
- Irregular margins
- Hypoechogenicity
- Microcalcifications
- Taller-than-wide shape
- Increased vascularity
- Laboratory tests, including TSH, free T4, and calcitonin levels, should be obtained to assess thyroid function and screen for medullary thyroid cancer.
- The FNA biopsy should be performed under ultrasound guidance to ensure accurate sampling of the hypermetabolic area, as the risk of malignancy is significantly higher in PET-avid thyroid nodules compared to incidentally discovered nodules.
- If the FNA result is indeterminate, molecular testing of the sample may help determine malignancy risk, and even benign cytology results in PET-avid nodules may warrant closer follow-up or consideration of repeat biopsy due to the higher risk of false negatives, as noted in the evaluation of metastatic patients where 18 FDG-PET scanning is used as a diagnostic and prognostic tool 1.
From the Research
Evaluation of Hypermetabolic Thyroid Nodules on PET Scan
- Hypermetabolic thyroid nodules are a significant finding on PET scans, with a substantial risk of malignancy 2, 3.
- The prevalence of malignant incidental focal hypermetabolic thyroid lesions detected by fluorine-18 fluorodeoxyglucose positron emission tomography (F-FDG-PET) is around 33.2% 2.
- A study of over 6000 patients found that the incidence of thyroid cancer in focal hypermetabolic thyroid lesions is 21.4% 3.
- The maximum standard uptake value (SUVmax) and thyroid to background (TL/TBG) ratio can help differentiate between benign and malignant lesions, with a TL/TBG ratio of more than 2.0 indicating a higher likelihood of malignancy 3.
Diagnostic Approach
- The diagnostic approach to hypermetabolic thyroid nodules typically involves ultrasound and fine-needle aspiration (FNA) biopsy 4, 3.
- The Bethesda Classification System is used to report FNA specimens, and molecular analysis of indeterminate FNA samples may help better discriminate between benign and malignant nodules 4.
- PET/CT is a sensitive and specific diagnostic procedure that can help characterize the risk of thyroid nodules and identify differentiated thyroid cancer (DTC) relapse early 5.
Management
- The management of hypermetabolic thyroid nodules depends on the clinical context and the results of diagnostic tests 5, 4.
- Surgery may be indicated for FNA findings of malignancy or indeterminate cytology with a high-risk clinical context, as well as for larger nodules with symptoms of mass effect or thyrotoxicosis 4.
- Radioactive iodine therapy may be considered for patients with unresectable or iodine-avid structural relapse 5, 6.