431 Letter: preoperative stratification by [18F]FDG-PET 9 futile diagnostic lobectomies with associated complications as well as financial and quality-of-life related sequelae [651]. Of these stratifying tests, molecular diagnostics and molecular imaging using [99mTc]Tc-MIBI and [18F]FDG seem the most promising. After the first, somewhat disappointing report of 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) imaging of thyroid tumours using a conventional Anger camera in 1988 [652], two follow-up studies in 1993 successfully showed that imaging of [18F]FDG using positron emission tomography (PET) could discern malignant from malignant nodules [653, 654]. Encouraged by our own pre-Bethesda experience [37], a meta-analysis of available literature [304] and modelled cost-effectiveness [53], it was concluded that a negative [18F]FDG-PET, obtained in 37% of patients, shows sufficient negative predictive value (NPV, 96%) to rule-out malignancy while being cost-effective, specifically for nodules larger than 15 mm when using the PET-systems available up to 2010. The remaining RoM of 3.6% was very comparable to that of a Bethesda II nodule, the definition of an “ideal rule-out test” according to actual guidelines [17]. Despite these findings, in their 2015 guidelines the American Thyroid Association does not routinely recommend [18F]FDG-PET imaging for the evaluation of thyroid nodules with indeterminate cytology (Recommendation 18: Weak recommendation) and therefore the guideline neither endorses nor discourages its use [17]. The reasoning was rather limited, moderate-quality evidence, likely fed by co-existent variable and sometimes conflicting data [312, 655-659]. Several causes have been attributed to the disappointing findings following initial promising work. These include improved cytology classification, inclusion of smaller lesions and shift from PET-only to hybrid PET-CT imaging. As earlier work was from the pre-TBSRTC-era, it is likely that improved stratification between Bethesda II and Bethesda V/VI may leave less aggressive histopathological diagnosis and thus less [18F]FDG-avid variants in the indeterminate cytology group [309]. Another explanation of decreasing NPV over time include the increasing number of smaller malignancies reported in studies. Indeed, false-negative [18F]FDG-PET-CT is often observed for sub-centimetre lesions [307, 308] and a positive relation between nodule size and NPV has been described [304]. Technical advances in PET-hardware and reconstruction algorithms have led to improvement in imaging resolution and signal-to-background ratio. This development, including the fact that most recent studies exclude sub-centimetre nodules and consider (sub)millimetric carcinomas a coincidence rather than a false-negative malignancy, however, have not led to less false negative cases and still malignancies up to 20 mm have been described to be [18F]FDG-nonavid [309]. The third cause, the transition from PET-only to hybrid PET/CT imaging, was specifically addressed by two recent meta-analysis, analysing over 1,000 nodules in over 20 independent cohorts. These show large ranges in observed sensitivities (0-94%, pooled: 73-74%) and specificities (41-91%, pooled: 56-58%) [658, 659]. A better performance of [18F]FDG-PET-only studies as compared to hybrid PET-CT studies is reported, mainly due to better sensitivity (pooled 95% versus 73%) but comparable specificity (pooled 58% versus 56%) [659]. NPVs should be interpreted with caution
RkJQdWJsaXNoZXIy MTk4NDMw