Top Line: What is the risk of marginal failure when smaller CTV margins are used to treat GBM?
The Study: The traditional CTV margins for GBM are relatively large and often result in a large volume of the brain receiving radiation. Patterns of failure studies have shown that most GBM failures are in-field and most are even within 1 cm of the original enhancing tumor. This has led many consensus groups to recommend as low as a 1 cm CTV margin. There are many out there, including several American Brain Tumor Consortium centers, that use even smaller 5 mm CTV margins. In this single arm, phase 2 trial, 98 patients receiving concurrent chemoradiation for GBM were treated with MR-LINAC enabled CTV margin reduction. The target included the enhancing tumor plus a 5 mm CTV margin, and T2 FLAIR abnormality could also be included at the treating physician’s discretion. They received either 60 Gy in 30 fractions (60%) or 40 Gy in 15 fractions (40%). All patients were treated on an MR-LINAC and underwent weekly online adaptation with contrast enhanced MRI. With a median follow up of 14.2 months, the rate of marginal failure was just 4%, which was well below the prespecified inferiority margin of 10%.
TBL: In this phase 2 study, leveraging an MR-LINAC to reduce CTV margins to 5 mm resulted in a low risk of marginal failure after definitive chemoradiation for GBM. | Detsky, Lancet Oncol 2026
The Study: The traditional CTV margins for GBM are relatively large and often result in a large volume of the brain receiving radiation. Patterns of failure studies have shown that most GBM failures are in-field and most are even within 1 cm of the original enhancing tumor. This has led many consensus groups to recommend as low as a 1 cm CTV margin. There are many out there, including several American Brain Tumor Consortium centers, that use even smaller 5 mm CTV margins. In this single arm, phase 2 trial, 98 patients receiving concurrent chemoradiation for GBM were treated with MR-LINAC enabled CTV margin reduction. The target included the enhancing tumor plus a 5 mm CTV margin, and T2 FLAIR abnormality could also be included at the treating physician’s discretion. They received either 60 Gy in 30 fractions (60%) or 40 Gy in 15 fractions (40%). All patients were treated on an MR-LINAC and underwent weekly online adaptation with contrast enhanced MRI. With a median follow up of 14.2 months, the rate of marginal failure was just 4%, which was well below the prespecified inferiority margin of 10%.
TBL: In this phase 2 study, leveraging an MR-LINAC to reduce CTV margins to 5 mm resulted in a low risk of marginal failure after definitive chemoradiation for GBM. | Detsky, Lancet Oncol 2026