A Canadian pilot study reported that FET-PET/MRI identified significantly larger metabolically active tumor volumes than conventional MRI in more than half of high-grade glioma patients. The finding points to a systematic blind spot in imaging-based planning that can affect radiotherapy targeting. The result is relevant to glioma care because treatment margins depend on how disease extent is visualized; metabolic imaging may refine target definition when MRI underestimates activity. The study design was presented as a pilot, but the signal supports ongoing interest in integrating molecular or metabolic imaging into planning workflows for brain tumors.