Technical Note: The first live treatment on a 1.0 Tesla inline MRI-linac.

Purpose: This work describes the first live imaging and radiation delivery performed on a prototype 1.0 T inline MRI-Linac system in a rat brain tumor model, which was conducted on 29 January 2019.

Methods: A human scale 1.0 T MRI-Linac was adapted to be suitable for animal studies via a specially constructed open 6-channel receiver radiofrequency (RF) coil. A Fischer rat injected with 9L glioma cells in the right hemisphere was imaged and irradiated at day 11 post surgery as part of a larger cohort survival study. The rat was anesthetized and positioned at the iscocenter of the MRI-Linac. Imaging was used to localize the brain and confirm the presence of a tumor following the administration of a gadolinium nanoparticle contrast agent. A single dose of 10 Gy was delivered using a 2.25 cm × 2.90 cm radiation field covering the whole brain and verified with radiosensitive film in situ. Real-time imaging was used throughout the irradiation period to monitor the animal and target position.

Results: The signal-to-noise ratio (SNR) measured in the rat brain was 38. Postcontrast imaging was able to demonstrate a tumor of 5 mm diameter in the upper right hemisphere of the brain approximately 45 min after administration of the nanoparticles. The radiation beam had no impact on SNR and images at the rate of 2 Hz were effective in monitoring both respiration and intrafractional motion. In vivo film dosimetry confirmed the intended dose delivery. The total procedure time was 35 min.

Conclusions: We have successfully used MRI guidance to localize and subsequently deliver a radiation field to the whole brain of a rat with a right hemispheric tumor. Real-time imaging during beam on was of sufficient quality to monitor breathing and perform exception gating of the treatment. This represents the first live use of a high field inline MRI-Linac.