Glioblastoma: Radiation treatment margins, how small is large enough?

Standard treatment for glioblastoma consists of surgical resection followed by radiation therapy with concurrent and adjuvant chemotherapy. Conventional radiation clinical treatment volumes include a 2- to 3-cm margin around magnetic resonance imaging or computed tomography enhancing abnormalities in the brain as well as a margin around the T2 or fluid-attenuated inversion recovery abnormality. However, there remains significant variability with respect to whether such extensive margins are necessary. Collectively, we as authors of this manuscript also use different margins, with A.G.W. employing European Organization for Research and Treatment of Cancer recommendations of a 2- to 3-cm margin on T1 enhancement for 60 Gy and M.P.M. using Radiation Therapy Oncology Group recommendations of 2 cm on T2 signal abnormality for the initial 46 Gy and 2.5-cm margin on T1 enhancement for a 14-Gy boost. Our experiences reflect the heterogeneity of margin definition and selection for this disease and underscore an important area of further research to minimize this variability. In this article, we review studies exploring recurrence patterns and outcomes in patients treated using both conventional and more limited margins. We conclude that treating to "smaller" margins does not alter recurrence patterns nor does it result in inferior survival, but whether this is because of the inherently limited benefit of radiation therapy in the first place, or whether it is truly because microscopic tumor control at larger distances is not an issue, remains unestablished.