Risk of Malignancy Associated with Focal Thyroid Uptake on Choline-PET/CT.

Introduction: The purpose of this study was to evaluate the pathological nature of focal thyroid uptake seen in Choline-PET/CT performed for prostate cancer.

Methods: The study was IRB-approved. All ¹¹C-Choline-PET/CT exam reports for studies performed between 01/01/2018 and 07/30/2021 in a single institution were retrospectively reviewed using a Softek Illuminate® InSight database. Exams with thyroid uptake on their final report were selected and a senior radiology resident reviewed these patient's medical records and images. Patients were excluded if they had surgery or ablation in the thyroid prior to the PET/CT, proven parathyroid nodules or if they lacked concurrent sonographic evaluation of the thyroid. Repeated PET/CT exams of the same patient were excluded.  PET images were analyzed visually and semi-quantitatively by measuring the maximum standardized uptake value (SUVmax) of the focal thyroid uptake. Available thyroid ultrasound images were evaluated, including exams performed just prior to the PET/CT, and when possible, associated Ti-RADS score and size were reviewed. Available cytology and pathology reports were reviewed, and statistical analyses were performed.

Results: Out of 10,047 sequential ¹¹C-Choline-PET/CT studies, 838 had final reports that included “thyroid” and 318 included “focal uptake” within or adjacent to the thyroid. A total of 74 patients had focal thyroid uptake and concurrent thyroid sonographic evaluation. 21 were diagnosed with presumed benign thyroid nodules based on the ultrasound features and 53 had further evaluation with biopsy. A total of 62 nodules were benign: 21 presumed benign on ultrasound, 39 benign thyroid nodules on pathology, 1 follicular hyperplasia and 1 Hürthle cell adenoma. 10 nodules were malignant: 1 follicular adenoma with papillary microcarcinoma, 7 papillary carcinomas, 1 MALT lymphoma and 1 malignancy with squamous cell characteristics. 2 nodules remained indeterminate: 1 Hürthle cell nodule and 1 follicular nodule with atypia not surgically removed. There was no significant difference between the SUV values of the benign and malignant groups (p>0.3). The average SUVmax of focal thyroid lesions was 4.5±1.7, for the benign lesions 4.8±1.6, for the indeterminate lesions 8.1±3.5 and for the malignant lesions 4.6±1.2.

Conclusions: Focal thyroid uptake on ¹¹C-Choline-PET/CT with a corresponding nodule on ultrasound had a 13.5% risk of malignancy in our institution. Therefore, consideration of further investigation with sonography and sonographic-guided FNA is warranted.