Tracer uptake in relation to histological grade (Table 2)
The median MET T/N ratios in grade Ⅱ (n = 12), grade Ⅲ (n = 24), and grade Ⅳ (n = 45) gliomas were 2.51 (interquartile range [IQR] 1.82–3.13), 5.03 (3.58–6.39), and 5.99 (4.64–7.39), respectively. There were significant differences in the median MET T/N ratios between grade Ⅱ and Ⅲ gliomas (P < 0.001) and grade Ⅱ and Ⅳ gliomas (P < 0.001), but not between grade Ⅲ and Ⅳ gliomas. The median FLT T/N ratios in grade Ⅱ, grade Ⅲ, and grade Ⅳ gliomas were 2.24 (IQR 1.69–2.62), 6.25 (4.59–8.69), and 15.24 (11.11–20.49), respectively. There were significant differences in the median FLT T/N ratios between grade Ⅱ and Ⅲ gliomas (P < 0.001) and grade Ⅲ and Ⅳ gliomas (P < 0.001).
Tracer uptake in relation to histopathological classification
The median MET T/N ratios in DA (n = 9), OD (n = 3), AA (n = 14), AO (n = 10), and GBM (n = 45) were 2.03 (IQR 1.83–2.97), 2.86 (2.51–3.23), 4.65 (3.25–5.99), 5.07 (4.72–7.21), and 5.99 (4.64–7.39), respectively. OD was omitted from the statistical analysis because of the small number of cases. There were significant differences of the median MET T/N ratios between DA and AA (P = 0.016), DA and AO (P = 0.009), and DA and GBM (P < 0.001; Fig. 1A). The median FLT T/N ratios in DA, OD, AA, AO and GBM were 2.35 (IQR 1.69–2.60), 2.14 (1.91–3.42), 7.16 (4.77–9.41), 5.82 (4.26–6.79), and 15.24 (11.11–20.46), respectively. There were significant differences in the median FLT T/N ratios between DA and AA (P = 0.002), DA and AO (P = 0.005), DA and GBM (P < 0.001), AA and GBM (P = 0.002), and AO and GBM (P < 0.001; Fig. 1B).
Tracer uptake in relation to IDH1 mutation status
In all gliomas, the median MET T/N ratios in IDH1-mutant and IDH1-wildtype tumours were 3.6 (IQR 2.84–5.59) and 5.91 (4.57–7.35), respectively. The median MET T/N ratio in IDH1-wildtype tumours was significantly higher than that in IDH1-mutant tumours (P < 0.001; Fig. 2A). The median FLT T/N ratio in IDH1-mutant and IDH1-wildtype gliomas were 4.18 (IQR 2.28–6.39) and 14.7 (8.98–20.38), respectively. Again, the median FLT T/N ratio in IDH1-wildtype tumours was significantly higher than that in IDH1-mutant tumours (P < 0.001; Fig. 2B).
There was a significant overlap of MET T/N ratios between IDH1-mutant and IDH1-wildtype tumours. On the other hand, the overlap of FLT T/N ratios between IDH1-mutant and IDH1-wildtype tumours was small. ROC analysis for differentiating IDH1-mutant tumours from IDH1-wildtype tumours showed that the area under the curve (AUC) of the FLT T/N ratio (AUC 0.911, 95% CI 0.847–0.975; Fig. 3B) was significantly larger than that of the MET T/N ratio (AUC 0.727, 95% CI 0.607–0.847; Fig. 3A) (P < 0.01). When the cutoff value of the FLT T/N ratio in the ROC curve was set at 6.74, the sensitivity for the differential diagnosis was 92.3%, and the specificity was 75.9%. On the other hand, the sensitivity for the differential diagnosis was 88.5%, and the specificity was 51.7% when the cutoff value of the MET T/N ratio was set at 3.72.
In the present study, more than 50% of the gliomas were grade Ⅳ GBMs, and almost all GBMs were IDH1-wildtype (43/45 cases). PET tracer uptake in GBMs is usually higher than that in other grades of gliomas, and this may influence the results in relation to IDH1 mutation status. Therefore, we conducted further examination of the uptake values in in 36 grade Ⅱ and Ⅲ gliomas. In this population, the median MET T/N ratios in IDH1-mutant and IDH1-wildtype tumours were 3.59 (IQR 2.87–5.28) and 5.14 (3.97–6.06), respectively. There was no significant difference in the median MET T/N ratios between IDH1-mutant and IDH1-wildtype tumours (Fig. 4A). In the same population, the median FLT T/N ratios in IDH1-mutant and IDH1-wildtype tumours were 3.43 (IQR 2.20–5.82) and 7.56 (6.00-12.94), respectively. The median FLT T/N ratio in IDH1-wildtype tumours was significantly higher than that in IDH1-mutant tumours (P < 0.01; Fig. 4B).
Tracer uptake in relation to histological grade in IDH1-mutant and IDH1-wildtype tumours
In the present study, there were only 2 IDH1-mutant GBMs among the 29 IDH1-mutant tumours. We therefore excluded the 2 IDH1-mutant GBMs from the analysis. In the remaining 27 IHD1-mutant tumours, the median MET T/N ratios were 2.86 (IQR 1.95–3.28) and 4.83 (3.48–6.39) for the grade Ⅱ (n = 11) and Ⅲ (n = 16) gliomas, respectively. The median MET T/N ratio in the grade Ⅲ gliomas was significantly higher than that in the grade Ⅱ gliomas (P = 0.002; Fig. 5A). In the same population, the median FLT T/N ratios in the grade Ⅱ and Ⅲ gliomas were 2.14 (IQR 1.69–2.64) and 5.36 (3.90–7.09), respectively. Again, the median FLT T/N ratio in the grade Ⅲ gliomas was significantly higher than that in the grade Ⅱ gliomas (P < 0.001; Fig. 5B).
In the 52 IDH1-wildtype tumours, there was only 1 IDH1-wildtype DA. We therefore excluded the DA from the analysis. In the remaining 51 IHD1-wildtype tumours, the median MET T/N ratios were 5.41 (IQR 4.11–6.20) and 6.19 (4.63–7.41) for the grade Ⅲ (n = 8) and Ⅳ (n = 43) gliomas, respectively. There was no significant difference of the median MET T/N ratios between grade Ⅲ and Ⅳ gliomas (Fig. 6A). In the same population, the median FLT T/N ratios in the grade Ⅲ and Ⅳ gliomas were 8.10 (IQR 6.58–14.53) and 15.65 (10.35–20.77), respectively. The median FLT T/N ratio in the grade Ⅳ gliomas was significantly higher than that in the grade Ⅲ gliomas (P = 0.029; Fig. 6B).