(18)F-Fluorothymidine PET-CT for resected malignant gliomas before radiotherapy: tumor extent according to proliferative activity compared with MRI

Abstract

Objective: To compare the presence of post-operative residual disease by magnetic resonance imaging (MRI) and [18F]fluorothymidine (FLT)-positron emission tomography (PET)-computer tomography (CT) in patients with malignant glioma and to estimate the impact of 18F-FLT PET on the delineation of post-operative target volumes for radiotherapy (RT) planning.

Methods: Nineteen patients with post-operative residual malignant gliomas were enrolled in this study. For each patient, 18F- FLT PET-CT and MRI were acquired in the same week, within 4 weeks after surgery but before the initiation of RT. The PET-CT and MRI data were co-registered based on mutual information. The residual tumor volume defined on the 18F-FLT PET (Vol-PET) was compared with that of gadolinium [Gd] enhancement on T1-weighted MRI (Vol-T1) and areas of hyperintensity on T2-weighted MRI (Vol-T2).

Results: The mean Vol-PET (14.61 cm3) and Vol-T1 (13.60 cm3) were comparable and smaller than the mean Vol-T2 (32.93 cm3). The regions of 18F-FLT uptake exceeded the contrast enhancement and the hyperintense area on the MRI in 14 (73.68%) and 8 patients (42.11%), respectively. In 5 (26.32%) of the 19 patients, Vol-PET extended beyond 25 mm from the margin of Vol-T1; in 2 (10.53%) patients, Vol-PET extended 20 mm from the margin of Vol-T2. Vol-PET was detected up to 35 mm away from the edge of Vol-T1 and 24 mm away from the edge of Vol-T2. In 16 (84.21%) of the 19 patients, the Vol-T1 extended beyond the Vol-PET. In all of the patients, at least some of the Vol-T2 was located outside of the Vol-PET.

Conclusions: The volumes of post-operative residual tumor in patients with malignant glioma defined by 18F-FLT uptake on PET are not always consistent with the abnormalities shown on post-operative MRI. Incorporation of 18F-FLT-PET in tumor delineation may have the potential to improve the definition of target volume in post-operative radiotherapy.

Fig 1. Comparison of tumor volumes defined by T1-weighted MRI with contrast enhancement and ¹⁸F-FLT PET.

For all 19 cases, the mean Vol-T2 was 32.93 cm³ (range, 3.3–106.60 cm³). The average summed volume Vol-(T2∪PET) was 36.36 cm³, and the average intersection Vol-(T2 ∩ PET) was 11.12 cm³. The average Vol-(T2 minus PET) was 21.78 cm³ (range, 0.8–83.20 cm³), and the average Vol-(PET minus T2) was 3.43 cm³ (range, 0–17.5 cm³).

Fig 2. Comparison of tumor volumes defined by T2-weighted MRI and ¹⁸F-FLT PET.

Based on the PET/CT and MRI/CT fusion images, the tumor contours derived from PET and MRI images were superimposed together on CT images and compared. In two patients (10.5%), the regions elevated between ¹⁸F-FLT uptake in PET images and the Gd enhancement in T1-weighted MR images corresponded exactly to each other. In 14 patients (73.68%), the ¹⁸F-FLT uptake was detected beyond the scope of Gd enhancement up to 35 mm from the tumor margin in MRI (range, 2–35 mm). In 5 (26.3%) of the 19 patients, ¹⁸F-FLT uptake extended beyond 25 mm from the margin of Gd enhancement. Additionally, in 16 patients (84.2%), the Gd enhancement was located outside the ¹⁸F-FLT uptake range (Table 3 and Fig. 3).

Fig 3. MRI and ¹⁸F-FLT PET images for a patient with glioblastoma (GBM).

Images were taken 21 days post-operatively and 2 days before radiotherapy. a. T1-weighted MRI with contrast enhancement. b. ¹⁸F-FLT PET images. c. CT image of PET-CT scan. Residual tumor regions defined by T1-MRI (red line) and ¹⁸F-FLT PET (yellow line) are superimposed on the CT image.

Fig 4. MRI and ¹⁸F-FLT PET-CT images from a patient with GBM.

Images were taken 21 days post-operatively and 2 days before radiotherapy. a. T2-weighted MRI. b. ¹⁸F-FLT PET image. c. CT image of PET-CT scan. Residual tumor regions defined by T2-MRI (blue line) and ¹⁸F-FLT PET (yellow line) are superimposed on the CT image.