Time-resolved dose reconstruction by motion encoding of volumetric modulated arc therapy fields delivered with and without dynamic multi-leaf collimator tracking

Abstract

Background. Organ motion during treatment delivery in radiotherapy (RT) may lead to deterioration of the planned dose, but can be mitigated by dynamic multi-leaf collimator (DMLC) tracking. The purpose of this study was to implement and experimentally validate a method for time-resolved motion including dose reconstruction for volumetric modulated arc therapy (VMAT) treatments delivered with and without DMLC tracking. Material and methods. Tracking experiments were carried out on a linear accelerator (Trilogy, Varian) with a prototype DMLC tracking system. A motion stage carrying a biplanar dosimeter phantom (Delta4PT, Scandidos) reproduced eight representative clinical tumor trajectories (four lung, four prostate). For each trajectory, two single-arc 6 MV VMAT treatments with low and high modulation were delivered to the moving phantom with and without DMLC tracking. An existing in-house developed program that adds target motion to treatment plans was extended with the ability to split an arc plan into any number of sub-arcs, allowing the calculated dose for different parts of the treatment to be examined individually. For each VMAT sub-arc, reconstructed and measured doses were compared using dose differences and 3%/3 mm γ-tests. Results. For VMAT sub-arcs the reconstructed dose distributions had a mean root-mean-square (rms) dose difference of 2.1% and mean γ failure rate of 2.0% when compared with the measured doses. For final accumulated doses the mean rms dose difference was 1.6% and the γ failure rate was 0.7%. Conclusion. The time-resolved motion including dose reconstruction was experimentally validated for complex tracking and non-tracking treatments with patient-measured tumor motion trajectories. The reconstructed dose will be of high value for evaluation of treatment plan robustness facing organ motion and adaptive RT.

Acknowledgements

We gratefully thank Drs Patrick Kupelian (University of California, Los Angeles) and Katja Langen (MD Anderson Cancer Center, Orlando) for the prostate trajectories, Drs Yelin Suh (MD Anderson Cancer Center) and Sonja Dieterich (University of California, Davis) for the lung tumor trajectories, Görgen Nilsson and Peter Münger (Scandidos) for modifications of the Delta4PT software to allow export of time-resolved dose data, and Roman Iustin and Andreas Bergqvist (Micropos Medical AB) for modifications of the RayPilot system to allow integration with the tracking program.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

This work was supported by NCI Grant R01CA93626 and by The Danish Cancer Society, CIRRO (The Lundbeck Foundation Center for Interventional Research in Radiation Oncology), The Danish Council for Strategic Research, and Varian Medical Systems. Aarhus University Hospital, Denmark, received financial support through a research agreement with Varian Medical Systems, Palo Alto, CA.