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Dosimetric Impact of Respiratory Motion and Photon Beam Energy On VMAT Based Lung SBRT Treatment

A Licon

A Licon*, H Parenica , Y Li , N Kirby , D Saenz , K Rasmussen , N Papanikolaou , S Stathakis , University of Texas HSC SA, San Antonio, TX


WE-RAM2-GePD-T-3 (Wednesday, August 2, 2017) 10:00 AM - 10:30 AM Room: Therapy ePoster Lounge

Purpose: To evaluate MLC interplay during volumetric modulated arc therapy (VMAT) and quantify the effect on the dose delivered to the target.

Methods: A CIRS Dynamic Thorax Phantom was used with a 2 cm spherical target. This phantom has the ability to replicate a three-dimensional target motion representing the breathing of a patient. A 4DCT was acquired while the phantom was in motion to track the movement of the target. The scans were used to create a maximum intensity projection (MIP) image set to create the ITV/PTV. A treatment plan was optimized for 3 separate linear accelerators with different MLC thickness. The plans consisted of the following energies; 6 MV, 6X FFF, 10 MV, and 10X FFF depending on the linac make and model. During delivery the amplitude of motion was kept constant while the frequency was changed for the same breathing pattern. Measurements were made with ionization chambers at the center of the target and with film. A reference measurement was acquired with the phantom stationary for comparison.

Results: Preliminary results showed that the 6X FFF delivered with an ELEKTA VersaHD has the least differences when compared to the stationary delivery with the largest percent difference being 3.5%, while the 6MV plan delivered with a VARIAN 600CD Millennium 80 had the largest difference of 11.2%. Similar differences were observed against the calculated doses in the target. One plan was created as a Dynamic Conformal Arc Therapy (DCAT) which had the best agreement between the calculated and delivered dose.

Conclusion: MLC interplay effect can deteriorate the optimized dose when the target is in motion. The decreased time of the delivered dose when treating with flattening filter free beams can reduce the magnitude of the discrepancy from calculations. In the future, the effect of the target will be studied and analyzed.

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