For radiotherapy of high-grade glioma, dose escalation to hypercellular tumour could improve local control, but changes during treatment might necessitate target volume adaptation. Since hypercellular tumour causes low apparent diffusion coefficient (ADC) values, volumetric changes in low-ADC regions were quantified using near-daily MR-Linac imaging to evaluate adaptation necessity. Low-ADC regions increased in volume (median extremal change: 12.2%) and changed rapidly for certain patients (maximum growth/shrinkage rate: 7.1/9.1% per day). Low-ADC regions changed more in magnitude and grew more rapidly for resected tumours than intact ones. These findings imply that adaptation may be required for dose escalation to hypercellular glioma regions.
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