Intrafraction Accuracy and Efficiency of a Surface Imaging System for Deep Inspiration Breath Hold in Ablative Pancreatic Cancer Treatment

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Beam gating with deep inspiration breath hold (DIBH) has been widely used for motion management in radiotherapy. It commonly relies on some external surrogate for internal target motion, while the exact internal motion is unknown. In this study, we tracked internal targets and characterized the residual motion during DIBH treatment for pancreatic cancer patients guided by a surface imaging system. We also report the treatment time statistics.Eight consecutive patients with pancreatic cancer treated with surface-guided DIBH volumetrically-modulated arc therapy in 2019-2020 were selected for this study, each with some radiopaque markers (fiducial markers or surgical clips) implanted near or inside the target. Five patients were treated in 25 fractions, two were treated in 15 fractions, and one was treated in 10 fractions. The surface imaging system monitored a three-dimensional surface with gating thresholds of 3 mm translation and 3° rotation. During dose delivery, kV images were automatically taken every 20˚ or 40˚ gantry rotation, from which internal markers were identified. The marker displacement from their initial positions and the residual motion amplitudes were calculated. For the analysis of treatment efficiency, the treatment time of every session was obtained from imaging and treatment timestamps recorded at the treatment console.Even though the external surface was limited to 3 mm / 3°, significant residual internal target motion was observed for some patients. The residual motion ranged 3-21 mm. The average target displacement relative to the initial position was 0-3 mm for the cohort. The magnitude of instantaneous displacement was > 5 mm in 13% of the images analyzed. The average treatment time was 16 min with standard deviation 3 min.Internal target motion may differ significantly from external surrogate motion in DIBH treatment, even with surface image guidance. Radiographic verification of target position at the beginning and during each fraction is essential for accurate delivery of radiotherapy.