Purpose: Modern motion management strategies rely on geometric tolerances as a substitute for dosimetric tolerances since the effect of motion on delivered dose is generally unknown at treatment. Clinical outcome is linked to dose accuracy, and so the direct use of dose-based tolerances is desirable. This work evaluates the accuracy of a new real-time dose reconstruction method for dose guided motion management during pancreas stereotactic body radiation therapy (SBRT).
Methods: Motion-inclusive dose reconstruction was performed retrospectively for five pancreas SBRT VMAT plans using the research software DoseTracker. Dose errors were introduced through six static isocenter shifts, creating changes to CTV D95% of (-14.7 - +3.6) %-points. For five fractions (one patient), actual target motion from intrafraction kV imaging (@0.3 Hz) of implanted fiducials was used for dose reconstruction. DoseTracker was benchmarked against a clinical treatment planning system (TPS). The root-mean-square error (RMSE) of the dose differences were calculated for clinically relevant dose-volume metrics.
Results: Dose errors were accurately calculated by DoseTracker, with a RMSE relative to the TPS for the static shift cases of 1.4% (CTV ΔD95%), 1.7% (duodenum ΔD0.5cc), 1.1% (stomach ΔD0.5cc) and 2.3% (small bowel ΔD0.5cc). For the case of patient sampled motion the interfraction RMSE relative to the TPS was 1.4% (CTV ΔD95%), 0.8% (duodenum ΔD0.5cc), 1.2% (stomach ΔD0.5cc) and 4.3% (small bowel ΔD0.5cc). The calculation time was on average (138.7 ± 47.1) ms (±2SD).
Conclusion: A method for fast calculation of motion-induced dose errors was validated against a clinical TPS for pancreas SBRT cases. The accuracy with which dose errors were calculated was of a clinically acceptable magnitude, and calculation speed was adequate for real-time application. It is thus feasible to implement DoseTracker as a tool to report dose error information for critical structures in real time as part of a dose guided motion management strategy.
Funding Support, Disclosures, and Conflict of Interest: Funding support was provided by Varian Medical Systems.