**1. Introduction**

Differentiated thyroid cancer (DTC) represents about 1% of all malignant tumors; moreover, it is the most frequent form of endocrine carcinoma and is usually characterized by good prognosis [1–4]. In recent years, its incidence has been growing due to the increasing use of needle aspiration and thyroid ultrasound [5–7].

The role of nuclear medicine in the diagnostic and therapeutic work-up of DTC is pivotal. In fact, nowadays, exams performed with 131I are fundamental for the staging, the restaging, and the therapy of this carcinoma [4,8].

In recent years, we have been continuously experiencing an increase in the use of positron emission tomography/computed tomography (PET/CT) with 18F-fluorodeoxyglucose ( 18F-FDG) for the evaluation of various pathologies, both neoplastic and inflammatory. In

**Citation:** Dondi, F.; Pasinetti, N.; Gatta, R.; Albano, D.; Giubbini, R.; Bertagna, F. Comparison between Two Different Scanners for the Evaluation of the Role of 18F-FDG PET/CT Semiquantitative Parameters and Radiomics Features in the Prediction of Final Diagnosis of Thyroid Incidentalomas. *J. Clin. Med.* **2022**, *11*, 615. https://doi.org/ 10.3390/jcm11030615

Academic Editors: Giovanni Vitale, Arnoldo Piccardo and Francesco Fiz

Received: 27 December 2021 Accepted: 25 January 2022 Published: 26 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

this context, even in the diagnostic work-up of DTC this hybrid imaging modality has a central role, in particular in the evaluation of patients with no evidence of 131I avid disease but a persistence of elevated thyroglobulin levels [4,9,10].

With the increasing use of 18F-FDG PET/CT in the clinical practice, we have also been experiencing an increase in the detection of thyroid incidentalomas (TI) [11–13]. TIs are defined as thyroid lesions detected at imaging studies performed for non-thyroid pathologies [14,15].

The precise evaluation of TIs is mandatory, given the non-negligible risk of presence of CTD in a high amount of these findings [16–18]. In this context, a lot of authors have tried to clarify the role of 18F-FDG PET/CT for the definition of the precise nature of TIs, in terms of malignancy or benignancy [4]. However, the role of some PET/CT semiquantitative parameters, such as standardized uptake value (SUV), metabolic tumor volume (MTV) and total lesion glycolysis (TLG) has not yet been fully clarified and the results in literature are really heterogeneous [4].

Furthermore, in recent years we have been appreciating an increase in the extraction of specific quantitative features from PET images, called radiomics or texture analysis. In this setting, the use of radiomics for the correct evaluation of every type of incidentalomas is waking increasing interest [19,20]. The case of TIs is not an exception and some works about the use of texture analysis for their correct classification have been produced [21–24]. However, similarly to semiquantitative parameters, the use of radiomics in this setting has given non-clarifying and initial results.

The aim of this retrospective study is to evaluate the role of semiquantitative PET/CT parameters and radiomics features for the correct classification of TIs discovered at 18F-FDG PET/CT scans. Furthermore, the impact of different PET/CT tomographs on texture analysis and on its ability to predict the final outcome is a fundamental part of this work.
