A real-time image-guided six-degree-of-freedom internal-external correlation framework for motion monitoring on a standard linear accelerator

Abstract Objective. Real-time six-degree-of-freedom (6DoF) tumour motion monitoring is important for accurate stereotactic body radiotherapy (SBRT) of thoracic and abdominal cancer sites, yet existing approaches either require specialised imaging hardware or continuous fluoroscopic imaging. This study presents the development and first experimental implementation of a real-time image-guided 6DoF internal-external correlation (6D-IEC) modelling framework on a standard linear accelerator. Approach. The 6D-IEC framework estimates 6DoF tumour motion using a state-augmented linear correlation model between a 1DoF external respiratory monitor and internal fiducial marker positions, updated with intrafraction kV images every 3 seconds. Performance was characterised through two complementary evaluations: a prospective experimental investigation on a robotic motion platform, providing independent ground truth across eight patient-measured lung and liver motion traces (3 regular, 5 irregular); and a patient data study using intrafraction imaging data from three patients treated in the TROG 17.03 LARK liver SBRT trial (NCT02984566). 6DoF geometric error was quantified for both evaluations; software processing latency was measured in the experimental investigation. Main results. In the experimental investigation, mean ± SD geometric errors were 0.1±1.0 mm, 0.3±1.9 mm, and −0.3±1.4 mm along the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) translational axes, and 0.4°±1.2°, −0.8°±1.0°, and −0.2°±0.8° around the corresponding rotational axes (rLR, rSI, rAP). Software processing latency was 190±80 ms. In the patient data study, errors were −0.5±0.7 mm, −1.2±2.5 mm, −0.1±1.0 mm (LR, SI, AP), and 1.1°±1.7°, −0.9°±1.5°, and −0.4°±1.6° (rLR, rSI, rAP). Significance. This work demonstrates that real-time 6DoF motion monitoring with clinically relevant accuracy is achievable on a standard linear accelerator without specialised hardware or continuous kV imaging. 6D-IEC infers tumour motion from an external signal between imaging updates, with accuracy contingent on internal-external correlation, but requires approximately 95% fewer intrafraction kV images than continuous fluoroscopic approaches. This has implications for broader access to real-time image-guided radiotherapy.