Imaging and clinical characteristics of incidental abnormal thyroid uptake on PSMA PET/CT.

Abstract

e18077 Background: To investigate the incidence, features, and clinical characteristics of unexpected prostate-specific membrane antigen (PSMA) uptake within the thyroid gland of patients with prostate cancer (PCa) who underwent F-18 DCFPyL PSMA PET/CT. Methods: We retrospectively reviewed a cohort of 1148 PCa patients who underwent PSMA PET/CT at our institution between 11/2021 - 09/2023. Scans with abnormal thyroid uptake were identified. Clinical data included patient demographics, PCa history, Gleason score, prostate specific antigen (PSA), ultrasonography (US), fine needle aspirate (FNA) / thyroidectomy, histopathology, and history of neck radiation or prior malignancy. Each scan was further analyzed with lesion-specific iso-contour method (42% cutoff) to obtain PET parameters (most prominent nodule SUVmax, SUVmean, PSMA volume, tumor lesion PSMA, contralateral gland SUVmax, SUVmax-nodule / SUVmax-contralateral gland ratio, most prominent PCa lesion SUVmax), PCa tumor distribution, and tumor volume. CT-related variables (nodule number, dominant nodule size, short/long axis ratio, CT-HU, integral total value) were also collected. Pearson's chi-square testing and Kruskal-Wallis testing was performed for categorical and nonparametric data respectively. Receiver operating characteristic (ROC) curves with area under curve (AUC) value were determined using IBM SPSS (version 29.0). Results: Of the 1148 PSMA PET/CTs, 60 (5.2%) had abnormal thyroid uptake (nodular: 58, diffuse: 2). Diffuse uptake was found to represent thyroiditis in both patients. 43 (56.7%) had an US, 20 (33%) were biopsied (11 benign, 5 indeterminate, 4 malignant), and 5 (8.3%) pursued thyroidectomy (3 malignant, 2 benign). 22 patients (36.7%) had histopathology data and 5 malignancies (2 PCa, 1 CLL, 2 primary thyroid) were identified. Nodule size was larger in those who had an US relative to those who did not. Among those with US data, no significant differences in clinical or histopathological parameters were seen. SUVmax (p: 0.01; AUC: 0.876, 95%CI: 0.731-1.0) and SUVmean (p:0.01; AUC: 0.871, 95%CI: 0.722-1.0) were higher in the malignant group. Nodule SUVmax/contralateral gland SUVmax ratio showed borderline significance (p = 0.05). Conclusions: Incidental PSMA PET uptake in the thyroid gland is seen in approximately 5% of patients. Focal uptake should prompt consideration of US and FNA due to the possibility of prostate cancer metastasis and thyroid cancer. SUVmax/mean is higher in malignant lesions.