Incidental abnormal thyroid 18F-FDG uptake as a proportion of all PET-CT scans and incidence of malignancy in focal abnormal thyroid 18F-FDG uptake in our patient group are both comparable with the available limited literature (Vassiliadi & Tsagarakis, 2011; Kao et al., 2012; Soelberg et al., 2012; Cohen et al., 2001; Kang et al., 2003; Chen et al., 2005; Chen et al., 2009; Yi et al., 2005; Ho et al., 2011; Kim et al., 2005; Chu et al., 2006), reiterating the importance of detecting and investigating focal thyroid uptake on 18F-FDG PET-CT. Around a quarter of focal thyroid uptake were malignant; hence, focal lesions should be investigated further, if clinically appropriate. A significant proportion of 18F-FDG PET-CT scans are performed for known/unknown malignancy, and further assessment of incidental thyroid lesions may not be possible for many reasons. Regardless, it is important to detect thyroid lesions in all patients in order to optimise appropriate clinical decision-making.
Some of the patients who underwent USS were deemed benign sonographically despite having focal thyroid tracer uptake. It is recommended that all thyroid tracer focal uptake, if clinically appropriate, are investigated further with an ultrasound and FNAC because of the increased risk of malignancy (Bae et al., 2009; Pencharz et al., 2018; Haugen et al., 2015). Currently, we do not have agreed local guidelines for investigation of focal thyroid uptake on 18F-FDG PET-CT, but based on current recommendations and findings in this study, it would be our future practice to investigate all focal lesions to undergo FNAC where clinically necessary.
Similar number of diffuse and focal thyroid tracer uptake were found in our study, and this is important to differentiate them as they have different outcomes for patients. Abnormal thyroid tracer uptake, diffuse or focal, also appears to be more in females, and this is comparable with current literature (Stangierski et al., 2014).
Our secondary aim was to compare SUVmax across thyroid malignancy histological subtypes. Follicular carcinoma has a higher mean, median and range of SUVmax than papillary carcinoma in this study. Soelberg et al. in their meta-analysis found mean SUVmax to be 6.9 g/ml in malignant lesions (Soelberg et al., 2012), lower than our findings for both follicular and papillary carcinoma but comparable with our median SUVmax. Our sample size for each malignancy subtype is low, and mean SUVmax and median SUVmax might be affected by this, so should be interpreted cautiously.
Our study found malignancies with a wide range of SUVmax values, implying a higher mean SUVmax does not necessitate malignancy. It is debatable if SUVmax can differentiate between benign and malignant lesions, but overall it is thought that mean SUVmax is lower in benign lesions compared to malignant lesions (Soelberg et al., 2012). It is also unclear if a specific SUVmax indicates malignancy and certain cutoffs have been stipulated. In our study, we were unable to confidently identify a cutoff point for SUVmax to identify malignancy due to considerable overlap in SUVmax values in both benign and malignant focal uptake lesions. This is echoed by the results of two smaller studies, which also showed a large overlap in SUVmax value between benign and malignant lesions (Brindle et al., 2014; Agrawal et al., 2015). With the findings in this study and other studies, it is difficult to confidently use mean SUVmax value alone in differentiating between benign and malignant thyroid lesions. There is a role for SUVmax in thyroid malignancy, but a specific defined value or range cannot be accurately established in differentiating between benign or malignant lesions.
There are a few limitations to our study that we acknowledge. Our biggest limitation was that we excluded nearly a third of patients with focal thyroid tracer uptake who were lost to our follow-up. Most of those patients were referred to us initially from regional centres, and their follow-up records or results were not available for our reference. Despite this, the study still showed the prevalence of incidental thyroid uptake on 18F-FDG PET-CT. Another limitation of our study is that only a word search for “thyroid” was done in our reports, and images were not reviewed. As described earlier, all our scans are double reported within a pool of 4 experienced consultants, two of whom specialise in thyroid imaging. Most of these patients undergo scans for known malignancy, and a significant proportion of these scans are reviewed again for MDT requirements. Furthermore, there is a dedicated discrepancy meeting to identify reporting errors. We strongly believe, given the above, that the possibility of missing unreported thyroid lesions in these scans to be very low.
