Comparative Analysis of Recurrent Glioblastoma Target Contours via 11C-Methionine, 68Ga-Prostate-Specific Membrane Antigen Positron Emission Tomography, and Magnetic Resonance Imaging: Implications for Precision Radiotherapy Planning.

PURPOSE: Glioblastoma (GBM) recurrence poses challenges in radiation therapy treatment planning because reirradiation has limited leeway needed for precise target delineation. Although effective radiotracers are emerging for treatment planning, comparisons of 11C-methionine positron emission tomography (MET-PET), 68Ga-prostate-specific membrane antigen PET (PSMA-PET), and magnetic resonance imaging (MRI) for contouring recurrent GBMs are lacking in the literature. This case study aimed to highlight the differences and similarities in target contours delineated from 3 examinations, aiming to raise doubts about the adequacy of current radiation therapy planning practices.

METHODS AND MATERIALS: A 37-year-old female patient with recurrent Isocitrate dehydrogenase (IDH)1/2 wild-type GBM underwent MRI, MET-PET, and PSMA-PET scans. Target delineations were performed, and volumes were compared using the Dice similarity coefficient, conformity index, and overlap volume, considering different planning target volume margins.

RESULTS: We found that MET-PET and MRI volumes showed superior agreement compared with PSMA-PET across all similarity parameters, indicating a more marked discrepancy between PSMA-PET and other modalities. Increasing planning target volume margins demonstrated progressive convergence in intervolume discrepancies. Notably, PSMA-PET delineated larger volumes extending beyond MRI-based volumes.

CONCLUSIONS: MRI alone may not suffice for target delineation in recurrent GBMs. PET imaging modalities offer complementary insights. Combined PET-MRI guidance could improve tumor boundary detection in target delineation for reirradiation. Prospective trials are necessary to ascertain its impact on patient outcomes.