Glioma Radiation Therapy on a High Field 1.5 MR-Linac: Workflow and Initial Experience

Abstract

The development of high-field strength MR-guided radiotherapy systems enables daily MR imaging, the opportunity for adaptive radiotherapy, and functional imaging acquisition. This study reports the workflow implementation, treatment times, and initial experience of glioma radiotherapy on the 1.5T MR-Linac (MRL).Selected glioma patients treated with concurrent chemoradiation (chemoRT) between October 2019 and August 2020 were identified from a prospective database. All patients were treated on the MRL with backup plans on conventional Linac if the MRL was unavailable. Workflow timings were recorded and online adaptive plans were generated using the Adapt-To-Position (ATP) workflow derived from a pre-beam T1-weighted MRI. Temporal variation within the FLAIR hyperintense region (FHR) was assessed by the Dice similarity coefficient (DSC) and relative FHR volumes, compared to the initial FLAIR contour at the first MRL fraction. Multi-parametric images were acquired on the MR-Linac during radiation treatment, including diffusion-weighted imaging (DWI), chemical exchange saturation transfer (CEST), magnetization transfer (MT), and blood oxygenation level dependent (BOLD) resting-state fMRI. The behavior of selected functional parameter values was investigated within the FHR at the imaged fractions.Ten high grade glioma patients completed chemoRT to a median dose of 60 Gy (range, 54-60 Gy) in 30 fractions (range, 30-33), receiving a total of 287 fractions on the MRL. The mean in-room time per fraction was 37.3 minutes (range, 24-51 minutes), and 42.9 minutes if including post-beam research imaging. Three patients (30%) required re-planning between fractions 15 to 19 due to progression of tumor and/or edema identified on daily MRL imaging. At the 10, 20, and 30 day post-first fraction time points, 3, 3, and 4 patients had a FLAIR volume that changed by at least 20% relative to the first MRL fraction, respectively. Multi-parametric images including DWI, CEST, MT, and BOLD resting-state fMRI were successfully acquired on the MR-Linac. The median apparent diffusion coefficients (ADC) within the FHR and volumes of FLAIR were significantly correlated when the data from all patients and time points were pooled (r = 0.68, P < 10-3).We report the first clinical series of glioma patients treated with chemoRT on the MRL. The workflow and treatment times were clinically acceptable, and daily online MRL imaging triggered adaptive re-planning for selected patients. Acquisition of multi-parametric imaging sequences was feasible on the MRL during routine treatment workflow. ADC parameter changes could be reliably tracked during chemoRT, and mature clinical outcomes data is anticipated to define the role of functional imaging in adaptive radiotherapy.C. Tseng: Advisory Board; Sanofi. H. Chen: Employee; North York General Hospital. J. Stewart: None. A. Lau: None. R. Chan: None. L.S. Lawrence: Student affiliated with University of Toronto; Sunnybrook Research Institute. M. Campbell: None. S.D. Myrehaug: Advisory Board; Novartis AG.H. Soliman: None. Z.A. Husain: Independent Contractor; RadOncQuestions LLC. Research Grant; Merck. Travel Expenses; Elekta; NIH Head and neck PULA task force. J. Detsky: None. P. Maralani: None. B.M. Keller: None. M.E. Ruschin: Patent/License Fees/Copyright; Elekta AB.A. Sahgal: Research Grant; Elekta.