The primary objective of this study is to evaluate the diagnostic performance of 18F-FDG PET/CT performed by normal care pathway in the identification of malignancy of undetermined resulting thyroid lesions at cytology (TIR3A and TIR3B) comparing the 18F-FDG PET/CT result with histological data.

The secondary objective is to identify possible PET/ultrasound/genetic/radiomics variables that can more accurately define the potential malignancy of undetermined nodules and create a predictive model of malignity fed by standard parameters (derived from the normal care path).

The identification of the prognostic value of 18F-FDG PET/CT in such patients setting could, in fact, make the PET useful in the future in the selection of patients for surgery/ follow-up.

To better stratify the malignancy risk of the resulting undetermined cytological lesions (TIR3A and TIR3B), it is proposed to perform an 18F-FDG PET/TC survey in these patients by which semiquantitative parameters are derived (SUVmax, SUVmean, SUVpeak, MTV, TLG) and perform radiomatic analysis. In the case where the above mentioned PET/CT survey has been recently performed, the survey will not be repeated but the images will be reviewed with visual and semi-quantitative analysis. The results will be compared with those derived from ultrasound and genetic analysis (currently proposed in cases of TIR3A), with the subsequent histological examination derived from surgery (currently indicated by the normal care path in TIR3B cases and in case of reconfirmation of TIR3A cytology to a second needle aspiration on the nodule). It is expected that an algorithm with multiple parameters combined can be created to correctly point towards surgery or a conservative attitude of follow up.

Inclusion Criteria:

• Patients with undetermined cytology (TIR3B and TIR3A candidates for surgery) and 18F-FDG PET/CT;
• Obtaining informed consent for data collection and processing.
• Patient aged 18 years

Exclusion Criteria:

- Pregnant/breastfeeding women

The study is addressed to all patients belonging to the U.O. of Nuclear Medicine of the IRCCS AOU of Bologna for execution of needle aspirate on thyroid nodules with undetermined cytology (TIR3A, TIR3B) and candidates for surgery from 1 February 2022 to 1 February 2027. These patients will be referred to 18F-FDG PET/CT survey at our Center. If the patient has already performed the 18F-FDG PET/CT survey at our Center (within 4 months of performing the needle aspiration), with guidelines, this survey will be included without repetition.

The expected duration of the scientific study (patient enrollment, data analysis, preparation of an article) is 6 years.