313 [18F]FDG-PET/CT to prevent futile surgery: a blinded, randomised controlled multicentre trial 4 Secondary outcomes Sensitivity, specificity, NPV, PPV, and benign call rate of [18F]FDG-PET/CT were 94.1% (95% CI, 80.3%- 99.3%), 39.8% (95% CI, 30.0%-50.2%), 95.1% (95% CI, 83.5%-99.4%), 35.2% (95% CI, 25.4%-45.9%) and 31.1% (95% CI, 23.3%-39.7%), respectively (Table 7). Two of 132 (1.5%) [18F]FDG-PET/CT scans were false-negative (both in the diagnostic surgery group). In both cases, the corresponding index nodules had caused extensive debate during the blinded interpretation of the histopathology (i.e., benign or malignant diagnosis). Case 1 was a 15 mm left-sided solitary nodule. On [18F]FDG-PET/CT, it was a well-defined, smooth, and hypodense nodule with [18F]FDG-uptake that was similar to its background upon visual assessment (Figure 3). As such, it was defined as [18F]FDG-negative. SUV max and SUVpeak values for this nodule were 2.5 g/cm3 and 2.0 g/cm3, respectively, as compared to a SUV max of 2.4 g/cm3 for the background of normal thyroid tissue. On histopathology, the neoplasm composed mainly of spindle cells and small areas with morphological characteristics of PTC or follicular adenoma. It showed focal positive staining for Galectin-3 and HBME-1 and diffuse strong immunoreactivity for TTF-1 and PAX-8. On next-generation sequencing, a point mutation in NRAS was detected. Differential diagnosis of this nodule included PTC or follicular adenoma with uncommon spindle cell metaplasia. This nodule was ultimately classified as a spindle cell PTC (TNM pT1b), after extensive assessment of the histopathology by dedicated thyroid pathologists from the University Medical Centre Groningen (Groningen, the Netherlands) and Radboud university medical centre (Nijmegen, the Netherlands), and including consultation with a pathologist of the University of Pittsburgh Medical Centre (Pittsburgh, PA, USA) (Figures 4 and 5). Case 2 was a 32 mm right-sided solitary nodule with a large cystic component. On [18F]FDG-PET/ CT, it was a well-defined, hypodense nodule surrounded by a relatively thick rim of solid tissue. The solid parts of the nodule, best assessed on the caudal side, were [18F]FDG-negative compared to the background (Figure 6). SUVmax and SUVpeak values for this nodule were 2.5 g/cm3 and 2.0 g/cm3, respectively, as compared to a SUVmax of 1.7 g/cm3 for the background of normal thyroid tissue. On histopathology, it was a predominantly cystic, non-invasive lesion with a follicular growth pattern (Figures 7 and 8). The solid parts of this lesion surrounding the cyst had a maximum diameter of 8 mm. On microscopy, the lesion had heterogeneous follicular aspects (Figure 8b) but also areas with inconclusive papillary nuclear features (Figure 8c). The follicular epithelial cells had slightly enlarged nuclei, most of which were round but some of which were oval with some nuclear overlap and nuclear grooves. The lesion showed positive staining for Galectin-3 and CK-19 but also thyroid peroxidase (TPO). Based on these observations, the differential diagnosis included a well differentiated tumour of uncertain malignant potential (WDT-UMP) or a non-invasive follicular neoplasm with papillary-like nuclear features (NIFTP). Additional next generation sequencing was ultimately performed during
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