Volumetric Parameter of 18F-FDG PET/CT in Characterizing Thyroid Incidentalomas

Thyroid incidentalomas are unsuspected thyroid lesions discovered during imaging performed for unrelated medical reasons. With the expanding use of 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) for cancer staging and follow-up, the detection of incidental thyroid nodules particularly those showing focal FDG uptake-has significantly increased (Are C et al., 2007; Kim TY et al.,2005). Although the overall prevalence of thyroid incidentalomas detected on PET/CT is relatively low (reported between 1.1% and 4%), the risk of malignancy among FDG-avid nodules is high, ranging from 26.7% to 64.6%, which is considerably greater than in non-FDG-avid nodules (Shi H et al.,2018; Makis W \& Ciarallo A, 2017;Choi JY et al., 2006).

Globally, thyroid cancer is one of the most common endocrine malignancies, with an increasing incidence rate over the past two decades (Sung H et al.,2021). In Egypt, recent data indicate that thyroid cancer accounts for approximately 2.6% of all new cancers, with a noticeable increase in detection largely attributed to the rise in imaging investigations (Ibrahim AS et al., 2014;GLOBOCAN, 2020). Despite this increase, clinical guidelines still lack aconsensus on how to best manage thyroid incidentalomas detected by PET/CT, especially in resource-limited settings.

Previous studies have explored various semi-quantitative PET parameters, particularly SUVmax, as potential indicators of malignancy. Malignant thyroid incidentalomas tend to show significantly higher SUVmax values than benign ones, but considerable overlap exists, limiting its standalone diagnostic value (Shi H et al., 2018; Kim BH et al., 2013).

Furthermore, integrating ultrasound characteristics such as microcalcifications, hypoechogenicity, irregular margins, and taller-than-wide shape, has shown promise in enhancing diagnostic accuracy (Haugen BR etal., 2016; Russ G et al., 2017). However, data on how well these PET/CT and ultrasound features correlate with final histopathology remain limited in Egypt and other low-resource settings.

In a large meta-analysis, the pooled risk of malignancy in FDG-avid thyroid incidentalomas was estimated at 34.6% (Bertagna F et al., 2012), but single-center studies report wide variability, which may be due to differences in patient selection, imaging thresholds, and follow-up protocols. Given the growing detection of FDG-avid thyroid nodules and the lack of clear, standardized management protocols in Egypt, there is a critical need for localized research to guide decision-making and reduce unnecessary surgeries or missed malignancies