Impact of [ ¹¹C]methionine positron emission tomography for target definition of glioblastoma multiforme in radiation therapy planning

Purpose: The purpose of this work was to define the optimal margins for gadolinium-enhanced T ₁-weighted magnetic resonance imaging (Gd-MRI) and T ₂-weighted MRI (T ₂-MRI) for delineating target volumes in planning radiation therapy for postoperative patients with newly diagnosed glioblastoma multiforme (GBM) by comparison to carbon-11-labeled methionine positron emission tomography ([ ¹¹C]MET-PET) findings. Methods and Materials: Computed tomography (CT), MRI, and [ ¹¹C]MET- PET were separately performed for radiation therapy planning for 32 patients newly diagnosed with GBM within 2 weeks after undergoing surgery. The extent of Gd-MRI (Gd-enhanced clinical target volume [CTV-Gd]) uptake and that of T ₂-MRI of the CTV (CTV-T ₂) were compared with the extent of [ ¹¹C]MET-PET (CTV - [ ¹¹C]MET-PET) uptake by using CT - MRI or CT - [ ¹¹C]MET-PET fusion imaging. We defined CTV-Gd (x mm) and CTV-T ₂ (x mm) as the x-mm margins (where x = 0, 2, 5, 10, and 20 mm) outside the CTV-Gd and the CTV-T ₂, respectively. We evaluated the relationship between CTV-Gd (x mm) and CTV - [ ¹¹C]MET-PET and the relationship between CTV-T ₂ (x mm) and CTV - [ ¹¹C]MET-PET. Results: The sensitivity of CTV-Gd (20 mm) (86.4%) was significantly higher than that of the other CTV-Gd. The sensitivity of CTV-T ₂ (20 mm) (96.4%) was significantly higher than that of the other CTV-T ₂ (x = 0, 2, 5, 10 mm). The highest sensitivity and lowest specificity was found with CTV-T ₂ (x = 20 mm). Conclusions: It is necessary to use a margin of at least 2 cm for CTV-T ₂ for the initial target planning of radiation therapy. However, there is a limit to this setting in defining the optimal margin for Gd-MRI and T ₂-MRI for the precise delineation of target volumes in radiation therapy planning for postoperative patients with GBM.