Purpose: To investigate the effects of different dose rates on the plan fidelity for a cardiac synchronized volumetric modulated arc therapy (CSVMAT) treatment.
Methods: A QUASAR body phantom with an acrylic film cassette placed in the centre receptacle and a wood insert placed in the peripheral receptacle was CT scanned and imported into the Eclipse treatment planning system. The film insert was contoured as the heart, the wood insert contoured as lung, and additional structures were added for the esophagus and great vessels. The planning target volume (PTV) was created near the centre of the heart. Plans were optimized to deliver 25 Gy to 100% of the PTV in two full arcs, using a 10MV FFF beam with dose rates of 1600, 2000, and 2400 MU/min. CSVMAT plans were then adapted from each original treatment plan and synchronized to an artificial heart rate increasing from 50 to 100 beats per minute. A Varian TrueBeam linear accelerator was used to deliver the original and CSVMAT plans, irradiating separate strips of Gafchromic EBT-XD film positioned horizontally at the isocentre within the QUASAR phantom. The dose distribution of each CSVMAT plan was compared with its original plan by performing a gamma analysis on the corresponding films using FilmQA Pro.
Results: Gamma comparison (2%/2mm criteria, global normalization to dose maximum, 10% low dose threshold) between the CSVMAT and original plans returned passing rates of 98.69%, 99.13%, and 86.22% for the 1600, 2000, and 2400 MU/min dose rates, respectively. The dose profile across the region of interest showed that the 2400 MU/min CSVMAT plan delivery was under-dosed compared to the original plan.
Conclusion: The CSVMAT technique on a TrueBeam linear accelerator can provide accurate treatment plan delivery up to a dose rate of 2000 MU/min. Further investigation as to delivery robustness is warranted.
Funding Support, Disclosures, and Conflict of Interest: Authors Steven Thomas, Kirpal Kohli, and Justin Poon have filed a patent application for CSVMAT.