Purpose: Compared to conventional rotating gantry linacs, a fixed-beam radiotherapy system would be smaller, more robust and more cost-effective, potentially improving global access to radiotherapy. The purpose of this work is to investigate real-time image-guided radiotherapy using a fixed-beam system.
Methods: A prototype fixed-beam system was developed as a proof-of-concept for a compact radiotherapy system. The prototype consists of a fixed vertical beam and a patient rotation system (PRS) designed to safely immobilize a patient and rotate about the superior-inferior (SI) axis. A deformable phantom was designed with a tumor target that moves under gravity to mimic anatomic motion during patient rotation. The phantom was placed in the PRS and rotated to four planned beam angles while fluoroscopic kV imaging tracked the 3D target position in real-time using implanted fiducial markers. At each treatment angle, the field was adapted prior to delivery using updated MLC leaf positions for left-right (LR) and anterior-posterior (AP) motion and couch shifts for SI motion. EBT3 radiochromic film was used to measure the dosimetric coverage of the target and assess the delivered dose distribution.
Results: Over a full rotation, the target in the deformable phantom moved sinusoidally up to 6.0, 9.6 and 1.5 mm in the LR, AP and SI directions respectively. The MLC leaf positions were moved up to 10.9 mm and the couch was shifted up to 1.0 mm to compensate for this motion. Gamma analysis of the delivered dose compared to the planned dose resulted in a 96.3% pass rate (global 3%/2 mm) for the real-time image-guided treatment compared to a 80.9% pass rate for the same treatment without image-guidance.
Conclusion: This work demonstrates that real-time image-guided treatments can be delivered with a fixed-beam radiotherapy system without the need for complex rotating gantries.
Funding Support, Disclosures, and Conflict of Interest: Nguyen was funded by Early Career Fellowships from Australia NHMRC and CINSW. OBrien was funded by CINSW Career Development Fellowship. Keall was funded by NHMRC Senior Principal Research Fellowship, is an inventor on patents related to KIM and NanoX, is a founder and shareholder of Leo Cancer Care Pty Ltd.