Volumetric analysis of 18F-FDG PET in glioblastoma multiforme: prognostic information and possible role in definition of target volumes in radiation dose escalation

UNLABELLED: The use of (18)F-FDG PET for brain tumors has been shown to be accurate in identifying areas of active disease. Radiation dose escalation in the treatment of glioblastoma multiforme (GBM) may lead to improved disease control. On the basis of these premises, we initiated a pilot study to investigate the use of (18)F-FDG PET for the guidance of radiation dose escalation in the treatment of GBM.

METHODS: Patients were considered eligible to participate in the study if they had a diagnosis of GBM, were at least 18 y old, and had a score of at least 60 on the Karnofsky Scale. Patients were treated with standard conformal fractionated radiotherapy (1.8 Gy per fraction, to 59.4 Gy), with volumes defined by MRI. At a dose of 45-50.4 Gy, patients underwent (18)F-FDG PET for boost target delineation. Final noncoplanar fields (3-4) were designed to treat the volume of abnormal (18)F-FDG uptake plus a 0.5-cm margin for an additional 20 Gy (2 Gy per fraction), to a total dose of 79.4 Gy. If no abnormal (18)F-FDG uptake was observed, treatment was stopped after the conventional course of 59.4 Gy. Age, Karnofsky score, MRI-based volumes, and (18)F-FDG PET volume were analyzed as prognostic variables for time to tumor progression (TTP) and overall survival. (18)F-FDG PET volumes and MRI-based volumes were compared to assess concordance.

RESULTS: For the 27 patients who could be evaluated, median actuarial TTP was 43 wk, and median actuarial survival was 70 wk. On univariate analysis, (18)F-FDG PET, T1-weighted MRI gadolinium enhancement (excluding nonenhancing resection cavity), and T2-weighted MRI volumes were significantly predictive of TTP. On multivariate analysis, only (18)F-FDG PET volume retained significance for predicting TTP. Similar results were obtained on analysis of these variables as prognostic factors for survival. When (18)F-FDG PET-based volumes were compared with MRI-based volumes, a difference of at least 25% was detected in all patients, with all but 2 having smaller (18)F-FDG PET volumes. Of patients in whom (18)F-FDG uptake was initially present but treatment subsequently failed, 83% demonstrated the first tumor progression within the region of abnormal (18)F-FDG uptake.

CONCLUSION: In comparison with MRI, (18)F-FDG PET defined unique volumes for radiation dose escalation in the treatment of GBM. (18)F-FDG PET volumes were predictive of survival and time to tumor progression in the treatment of patients with GBM.