Preliminary Investigation of Adaptive Glioblastoma Radiation Therapy Using the Integrated High-Field MRI-LINAC

Abstract

The MRL has the potential to visualize changes in glioblastoma (GBM) tumor volume over the course of radiotherapy. We hypothesize that the MRL can adapt for tumor volume changes observable with MRI at time of treatment. As part of a prospective ethics-board approved imaging study, MR images of GBM patients receiving 60 Gy in 30 fractions are acquired at 4 time points: Day0 (planning), Day10 (post ten fractions), Day20 and at 1-month follow-up. Using the T1-weighted post-gadolinium injection images, the enhancing volume indicating the gross tumor volume (GTV) was contoured by an experienced radiation oncologist at each time point for 3 of 20 planned patients. Relative and absolute volume changes in GTV were measured. Plan adaptation feasibility was tested in one case by simulating a new planning target volume (PTV) based on the changed GTV and then using the Adapt Plan workflow in the Monaco MRL research planning system version 5.19.02 (Elekta AB, Stockholm, Sweden) to warp the beam apertures to the new PTV. For 2 of the 3 patients analyzed the GTV shrunk by 14% and 20% at Day20 and 51% and 42% at 1 month when compared to the original GTV. For the third patient, an increase in GTV of 37% was observed at Day20. In the case of GTV reduction, the enhancing region tended to shrink in medially from within the brain towards the surgical entry point. The image set containing the 20% GTV shrinkage at Day20 was used to generate the adapted plan with the new PTV being 31% smaller than the original. The resulting beam apertures were correspondingly reduced to the shape of the new PTV by a mean of 22% (18cm2). The reduction in beam aperture area followed by warm-start optimization maintained coverage of the new PTV while reducing the maximum dose to the brainstem and optic chiasm by 37% and 39% respectively for the fractions that would receive the adapted plan. We are continuing to gather novel data related to volumetric changes in GBM tumor volume over the course of radiotherapy. Preliminary observations indicate unidirectional changes over time continuing to occur throughout and after the course of radiotherapy. Preliminary findings also indicate the feasibility of adapting for observed tumor changes and ongoing work is being undertaken to develop an adaptive strategy for the MRL that involves auto-segmentation and planning boost doses with similar or improved safety profile compared to our current standard.