RT-22 * MRI AND FET PET FOR RADIOTHERAPY PLANNING IN RECURRENT HIGH-GRADE GLIOMA (HGG) IN A PROSPECTIVE PHASE I TRIAL

Abstract

BACKGROUND AND PURPOSE: Re-irradiation of HGG at recurrence may be a viable treatment option for some patients. Positron emission tomography (PET) using the amino acid tracer, 18F-fluoro-ethyltyrosine (FET) may have higher specificity and sensitivity than MRI in this setting. Both FET PET and MRI were used for target delineation in a prospective trial. The purpose of this study was to evaluate the impact of FET PET on the radiotherapy target. MATERIAL AND METHODS: A phase I trial examined the safety of hypofractionated stereotactic re-irradiation at different dose levels. Patients in PS 0-2 with localized recurrence of HGG and no other treatment options were eligible. With MRI, tumor was manually contoured using T1 contrast enhanced sequences. For PET, a threshold of 1.6x mean in background was used in a semi-automatic segmentation procedure. The planning target volume for treatment was the combination of both tumor volumes (MRI and PET) plus a 2 mm margin. Matlab software was used for spatial analyses. (Trial identifier: NCT02025231). RESULTS: Twenty-eight patients were included (GBM: n = 22; grade 3 glioma: n = 6). All patients had received radiotherapy and temozolomide, and 61 % had previously received bevacizumab. Median tumor volume defined by MRI and PET was 34 cm3 (range: 0-230 cm3) and 24 cm3 (range:0-214 cm3), respectively. Using PET increased the total gross tumor volume by a median of 10 cm3 (range:0-77 cm3), compared to MRI alone. The median longest distance from the MRI-target to encompass the PET-target was 10 mm (range: 0.6 to 43 mm). CONCLUSION: Tumor volumes defined by MRI and PET were spatially separated by up to 43 mm indicating that large portions of PET positive tumor would be missed if the target was solely based on MRI (T1 + contrast). FET PET may be useful for tumor delineation in re-irradiation of HGG.