**2. Materials and Methods**

#### *2.1. Patients Selection*

We retrospectively analyzed the 18F-FDG PET/CT scans performed in our center between January 2012 and December 2020 in order to find presence of TIs. All of the patients performed PET/CT exams for staging or restaging purpose of various diseases, but no one had a previous history of DTC. Specifically, 82 patients suffered from lymphoma, 19 from carcinomas of the head and neck, 51 from lung cancer, 6 from fever of unknown origin, 12 from vasculitis, 38 from breast cancer, 3 from esophageal cancer, 2 from ovarian cancer, 5 from colorectal cancer and 2 from endocarditis, while 1 patient performed the examination in order to characterize a formation of the right adrenal gland.

Tis were defined as focal uptakes of 18F-FDG inside the thyroid gland with an uptake higher than the background uptakes. Given the fact that diffuse uptakes on thyroid gland are usually expression of benign conditions, they were excluded from the study [4]. Furthermore, other inclusion criteria were the presence of an ultrasound follow-up of at least 1 year for suspected benignant uptakes and the execution of a cytological evaluation and/or histological examination for suspected malignant uptakes. A total of 237 patients were therefore included in the study and data about the lobe of TIs and ultrasound dimension were collected.
