Accuracy and precision of apparent diffusion coefficient measurements on a 1.5 T MR-Linac in central nervous system tumour patients.

Background and Purpose: MRI linear accelerators (MR-Linacs) may allow treatment adaptation to be guided by quantitative MRI including diffusion-weighted imaging (DWI). The aim of this study was to evaluate the accuracy and precision of apparent diffusion coefficient (ADC) measurements from DWI on a 1.5 T MR-Linac in patients with central nervous system (CNS) tumours through comparison with a diagnostic scanner.
Materials and Methods: CNS patients were treated using a 1.5 T Elekta Unity MR-Linac. DWI was acquired during MR-Linac treatment and on a Philips Ingenia 1.5 T. The agreement between the two scanners on median ADC over the gross tumour/clinical target volumes (GTV/CTV) and in brain regions (white/grey matter, cerebrospinal fluid (CSF)) was computed. Repeated scans were used to estimate ADC repeatability. Daily changes in ADC over the GTV of high-grade gliomas were characterized from MR-Linac scans.
Results: DWI from 59 patients was analyzed. MR-Linac ADC measurements showed a small bias relative to Ingenia measurements in white matter, grey matter, GTV, and CTV (bias: -0.05 ± 0.03, -0.08 ± 0.05, -0.1 ± 0.1, -0.08 ± 0.07 μm ² /ms). ADC differed substantially in CSF (bias: -0.5 ± 0.3 μm ² /ms). The repeatability of MR-Linac ADC over white/grey matter was similar to previous reports (coefficients of variation for median ADC: 1.4%/1.8%). MR-Linac ADC changes in the GTV were detectable.
Conclusions: It is possible to obtain ADC measurements in the brain on a 1.5 T MR-Linac that are comparable to those of diagnostic-quality scanners. This technical validation study adds to the foundation for future studies that will correlate brain tumour ADC with clinical outcomes.
Competing Interests: Conflicts of Interest Dr. Arjun Sahgal: Advisor/consultant with Abbvie, Merck, Roche, Varian (Medical Advisory Group), Elekta (Gamma Knife Icon), BrainLAB, and VieCure (Medical Advisory Board); Past educational seminars with Elekta AB, Accuray Inc., Varian (CNS Teaching Faculty), BrainLAB, Medtronic Kyphon; Research grant with Elekta AB; Travel accommodations/expenses by Elekta, Varian, BrainLAB. Dr. Chia-Lin Tseng: Travel accommodations/expenses & honoraria for past educational seminars by Elekta; belongs to the Elekta MR-Linac Research Consortium; advisor/consultant with Sanofi. Dr. Sten Myrehaug: Received research support from Novartis AG, honoraria from Novartis AG and Ipsen and travel support from Elekta. None related to this work. Dr. Mark Ruschin: Co‐inventor of and owns associated intellectual property specific to the image‐guidance system on the Gamma Knife Icon. None related to this work. Mrs. Mikki Campbell: Received honorarium for past educational seminars with Elekta AB; received research grants with Elekta AB; and travel accommodations/expenses by Elekta; belongs to the Elekta MR Linac Research Consortium. Dr. Brige Chugh: Previously held an industry partnership grant with Modus QA.
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