Room: Davidson Ballroom A
Purpose: Compared to conventional rotating gantry linacs, a fixed-beam linac with a rotating patient would be smaller, more robust and more cost effective. However, rotating a patient during radiotherapy introduces additional motion due to deformation which must be compensated for. In this work, we demonstrate the use of Kilovoltage Intrafraction Monitoring (KIM) and MLC tracking to account for target motion during rotation.
Methods: A miniature prototype of a fixed-beam system was built to rotate a phantom about the superior-inferior axis. A high dose IMRT treatment was delivered in two different configurations: rotating gantry with stationary phantom (conventional treatment) and stationary gantry at 0Â° with rotating phantom (fixed-beam treatment). In the fixed-beam treatment, static and dynamic movement were introduced in the phantom to simulate motion. KIM, which measures the 3D position using kV projections, and MLC tracking, which reshapes the beam based on the measured position, were used to compensate for the motion. The dose distribution for each beam configuration was measured using EBT3 film and compared using 2%/2 mm gamma analysis.
Results: Gamma analysis between conventional and fixed-beam treatments without motion resulted in 100% gamma pass rate, indicating dosimetric equivalence between these two beam configurations. In the presence of target motion, the dose distribution using MLC tracking had 99.0% or better pass rate compared to the fixed-beam treatment without motion and 98.7% or better pass rate when compared to the conventional treatment.
Conclusion: MLC tracking can successfully modulate a fixed radiation beam to compensate for target motion to maintain the desired dose distribution around a rotating target. This work enables further development of fixed-beam radiotherapy systems, where high quality motion adapted treatments can be delivered to patients without the need for linacs with complex rotating gantries.
Funding Support, Disclosures, and Conflict of Interest: Co-author Keall is an inventor on several pending patents related to KIM and MLC tracking. Co-authors Feain and Keall are inventors on several pending patents Nano-X technology and are founders and shareholders of Nano-X Pty Ltd, which is developing a fixed beam radiotherapy system.