ESTRO 2024 - Abstract Book


Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

The MLC-tracking system investigated was Synchrony on Radixact (Accuray). It records a sequence of 2D-kV images at varying angles, uses the three most recent images to estimate the current tumor position and adjusts the beam to account for the movement. Imaging is done up to six times per rotation with one fraction comprising of around 17 rotations in total for a prostate cancer treatment. In a series of controlled phantom experiments, Synchrony estimated the motion of a programmable 3D motion phantom (Delta4/Hexamotion) with three implanted fiducials that reproduced five previously published prostate trajectories [2]. For independent validation of the Synchrony system, the single-imager probability-based localization method of Kilovoltage Intrafraction Monitoring (KIM) [3] was adapted to the Radixact geometry and used to estimate the 3D phantom trajectories from the 2D-kV images acquired by Synchrony. The Synchrony- and KIM estimated trajectories were then compared to the programmed trajectory of the phantom. For clinical validation in patients, KIM retrospectively estimated the clinical tumor trajectories at a total of 25 fractions for four prostate cancer patients from a prospective trial using the 2D-kV images acquired by Synchrony during treatment. The KIM-estimated trajectories were then compared to the trajectories estimated by Synchrony. The observed motion was also compared to the concurrent values of Synchrony’s uncertainty metric.


KIM and Synchrony both had sub-millimeter root-mean-square differences (RMSD) between their estimated trace and the programmed phantom motion in all three directions (Table 1). The RMSD between KIM and Synchrony was also sub-millimeter for phantom experiments while the 3D RMSD was 1.19 mm for prostate cancer patients. Since KIM only uses one image (rather than three) to estimate the mean 3D position it responded faster than Synchrony to abrupt prostate motion (Fig 1). Furthermore, abrupt motion resulted in strong oscillations in Synchrony-estimated LR position, which were reflected in high values of the system's uncertainty metric (Fig 1-2). Similar oscillation did not occur in the KIM-estimated motion.

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