Room: Karl Dean Ballroom A1
Purpose: Experimental verification of 4D Monte Carlo (MC) simulations of dose delivery to a moving/deforming anatomy.
Methods: A deformable, tissue-equivalent lung phantom was constructed. The phantom holds a removable plug containing an elastic tumor. Lucite beads were injected as landmarks throughout the phantom. A piston attached to a DC motor produced a sinusoidal motion of 1.37 cm peak-to-peak amplitude at the center of the tumor in the SI direction.Irradiations were carried out on an Elekta Infinity linac with the phantom in stationary and moving/deforming states. Dose profiles within the plug were measured using calibrated EBT3 film. Point dose and position measurements inside and outside (top and bottom surfaces of the plug) the tumor were performed using RADPOS 4D dosimetry system. Square static field and VMAT plans were created on the end-of-inhale CT scans of the phantom in Monaco V.5.11.01 to cover the tumor. A validated BEAMnrc model of our 6MV linac was used for all dose simulations with DOSXYZnrc and 4DdefDOSXYZnrc user codes for stationary and moving/deforming anatomies, respectively. Delivery log files were used to generate simulation input files. For 4D simulations, deformation vectors were obtained by deformably registering 4DCT scans of the end-of-exhale to the end-of-inhale states using Velocity AI. Deformation vectors and phantom motion traces recorded by RADPOS were used to model the phantom motion.
Results: The phantom motion was reproducible within 1 mm. Simulated dose profiles agreed within 2%/2 mm with film for over 98% and 94% of points on the stationary and moving/deforming anatomies, respectively. Agreement of simulated and measured dose values was within 2% and 5% inside and outside the tumor, respectively.
Conclusion: Our 4DMC simulations accurately calculate dose delivered to a moving/deforming anatomy. The ongoing work is to modify the phantom and verify the accuracy of our simulations with irregular respiratory motion patterns.
Funding Support, Disclosures, and Conflict of Interest: This work has been supported by funding from the Ontario Consortium for Adaptive Radiotherapy (OCAIRO) as well the Natural Sciences and Engineering Research Council (NSERC) of Canada.