Comparison of diagnostic accuracy of 18F-FDG PET, 123I-IMT- and 99mTc-MIBI SPECT. Evaluation of tumour progression in irradiated low grade astrocytomas

Aim was to evaluates the diagnostic accuracy of the SPECT-tracers 3-123I-α-methyl-L-tyrosine (IMT) and 99mTc(I)-hexakis(2-methoxyisobutylisonitrile) (MIBI) as well as the PET-tracer 2-18F-2-deoxyglucose (FDG) for detecting tumour progression in irradiated low grade astrocytomas (LGA). Patients, methods: we examined 91 patients (56 males; 35 females; 44.7 ± 11.5 years), initially suffering from histologically proven LGAs (mean WHO grade II) and treated by stereotactic radiotherapy (59.0 ± 4.6 Gy). On average 21.9 ± 11.2 months after radiotherapy, patients presented new Gd-DTPA enhancing lesions on MRI, which did not allow a differentiation between progressive tumour (PT) and non-PT (nPT) at this point of time. PET scans (n = 82) were acquired 45 min after injection of 208 ± 32 MBq FDG. SPECT scans started 10 min after injection of 269 ± 73 MBq IMT (n = 68) and 15 min after injection of 706 ± 63 MBq MIBI (n = 34). Lesions were classified as PT and nPT based on prospective follow-up (clinically, MRI) for 17.2 ± 9.9 months after PET/SPECT. Lesion-to-normal ratios (L/N) were calculated using contra lateraly mirrored reference regions for the SPECT examinations and reference regions in the contra lateral grey (GM) and white matter (WM) for FDG PET. Ratios were evaluated by Receiver Operating Characteristic (ROC) analysis. Results: in the patient groups nPT and PT, L/N ratios for FDG (GS) were 0.6 ± 0.3 vs. 1.2 ± 0.5 (p = 0.003), for FDG (WS) 1.2 ± 0.4 vs. 2.6 ± 0.4 (p < 0.001), for IMT 1.1 ± 0.1 vs. 1.8 ± 0.4 (p < 0.001) and for MIBI 1.6 ± 0.7 vs. 2.6 ± 2.2 (p = 0.554). Areas under the non-parametric ROC-curves were: 0.738 ± 0.059 for FDG (GS), 0.790 ± 0.057 for FDG (WS), 0.937 ± 0.037 for IMT and 0.564 ± 0.105 for MIBI. Conclusion: MIBI-SPECT examinations resulted in a low accuracy and especially in a poor sensitivity even at modest specificity values. A satisfying diagnostic accuracy was reached with FDG PET. Using WM as reference region for FDG PET, a slightly higher AUC as compared to GM was calculated. IMT yielded the best ROC characteristics and the highest diagnostic accuracy for differentiating between PT and nPT in irradiated LGA. (orig.)