Utilizing 18F-fluoroethyltyrosine (FET) positron emission tomography (PET) to define suspected nonenhancing tumor for radiation therapy planning of glioblastoma

Aim: The authors sought to evaluate the impact of 18F-fluoroethyltyrosine (FET) positron emission tomography (PET) on radiation therapy planning for patients diagnosed with glioblastoma (GBM) and the presence of suspected nonenhancing tumors compared with standard magnetic resonance imaging (MRI). Methods and materials: Patients with GBM and contrast-enhanced MRI scans showing regions suspicious of nonenhancing tumor underwent postoperative FET-PET before commencing radiation therapy. Two clinical target volumes (CTVs) were created using pre- and postoperative MRI: MRI fluid-attenuated inversion recovery (FLAIR) sequences (CTVFLAIR) and MRI contrast sequences with an expansion on the surgical cavity (CTVSx). FET-PET was used to create biological tumor volumes (BTVs) by encompassing FET-avid regions, forming BTVFLAIRand BTVSx. Volumetric analyses were conducted between CTVs and respective BTVs using Wilcoxon signed-rank tests. The volume increase with addition of FET was analyzed with respect to BTVFLAIRand BTVSx. Presence of focal gadolinium contrast enhancement within previously nonenhancing tumor or within the FET-avid region was noted on MRI scans at 1 and 3 months after radiation therapy. Results: Twenty-six patients were identified retrospectively from our database, of whom 24 had demonstrable FET uptake. The median CTVFLAIR, CTVSx, BTVFLAIR, and BTVSxwere 57.1 mL (range, 1.1-217.4), 83.6 mL (range, 27.2-275.8), 62.8 mL (range, 1.1-307.3), and 94.7 mL (range, 27.2-285.5), respectively. When FET-PET was used, there was a mean increase in volume of 26.8% from CTVFLAIRto BTVFLAIRand 20.6% from CTVSxto BTVSx. A statistically significant difference was noted on Wilcoxon signed-rank test when assessing volumetric change between CTVFLAIRand BTVFLAIR(P <.0001) and CTVSxand BTVSx(P <.0001). Six of 24 patients (25%) with FET avidity before radiation therapy showed focal gadolinium enhancement within the radiation therapy portal. Conclusions: FET-PET may help improve delineation of GBM in cases with a suspected nonenhancing component. This may result in improved radiation therapy target delineation and reduce the risk of potential geographical miss. We investigated the impact of 18F-fluoroethyltyrosine (FET) positron emission tomography (PET) on radiation therapy planning for patients diagnosed with glioblastoma (GBM) and a suspected nonenhancing tumor compared with standard magnetic resonance imaging. We performed volumetric analyses between clinical target volumes and respective biological target volumes using Wilcoxon signed-rank tests. FET-PET may help improve delineation of GBM in cases with a suspected nonenhancing component and reduce the risk of potential geographical miss.