First online real‐time evaluation of motion‐induced 4D dose errors during radiotherapy delivery

Purpose In radiotherapy, dose deficits caused by tumor motion often far outweigh the discrepancies typically allowed in plan‐specific quality assurance ( QA ). Yet, tumor motion is not usually included in present QA . We here present a novel method for online treatment verification by real‐time motion‐including four‐dimensional (4D) dose reconstruction and dose evaluation and demonstrate its use during stereotactic body radiotherapy ( SBRT ) delivery with and without MLC tracking. Methods Five volumetric‐modulated arc therapy ( VMAT ) plans were delivered with and without MLC tracking to a motion stage carrying a Delta4 dosimeter. The VMAT plans have previously been used for (nontracking) liver SBRT with intratreatment tumor motion recorded by kilovoltage intrafraction monitoring ( KIM ). The motion stage reproduced the KIM ‐measured tumor motions in three dimensions (3D) while optical monitoring guided the MLC tracking. Linac parameters and the target position were streamed to an in‐house developed software program (DoseTracker) that performed real‐time 4D dose reconstructions and 3%/3 mm γ ‐evaluations of the reconstructed cumulative dose using a concurrently reconstructed planned dose without target motion as reference. Offline, the real‐time reconstructed doses and γ ‐evaluations were validated against 4D dosimeter measurements performed during the experiments. Results In total, 181,120 dose reconstructions and 5,237 γ ‐evaluations were performed online and in real time with median computation times of 30 ms and 1.2 s, respectively. The mean (standard deviation) difference between reconstructed and measured doses was −1.2% (4.9%) for transient doses and −1.5% (3.9%) for cumulative doses. The root‐mean‐square deviation between reconstructed and measured motion‐induced γ ‐fail rates was 2.0%‐point. The mean (standard deviation) sensitivity and specificity of DoseTracker to predict γ ‐fail rates above a given threshold was 96.8% (3.5%) and 99.2% (0.4%), respectively, for clinically relevant thresholds between 1% and 30% γ ‐fail rate. Conclusions Real‐time delivery‐specific QA during radiotherapy of moving targets was demonstrated for the first time. It allows supervision of treatment accuracy and action on treatment discrepancy within 2 s with high sensitivity and specificity.