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Time-Weighted 4D Dose Accumulation Using a Respiratory Motion Model On Tomotherapy

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D O'Connell

D O'Connell*, D Low , P Lee , X Qi , UCLA, Los Angeles, CA


SU-I-GPD-T-518 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Respiratory motion can potentially affect the accuracy of dose delivery to lung tumors. We aimed to examine differences between dose calculated on a single image and a set of respiratory-correlated images (4DCT). In contrast to other 4D calculation techniques, a breathing motion model was used to perform a time-weighted accumulation

Methods: A previously published model-based CT acquisition and analysis technique, involving fast helical free-breathing scanning with simultaneous respiratory surrogate monitoring and deformable image registration, was used to generate breathing-gated images of lung SBRT patients for treatment planning. Two plans were created for a Lung SBRT patient using different jaw settings on TomoTherapy: 1 cm fixed 2.5 cm dynamic. For each plan, dose was calculated on a single static free-breathing CT image, as well as 8 4DCT images. Dose-volumes on the 4DCT images were accumulated in two ways: simple deformation followed by averaging a time-weighted accumulation using a weighting factor for each gated image based on the relative amount of time spent at that breathing phase.

Results: Mean and maximum dose to the PTV remained within 1% for calculated dose delivery. For the 1 cm jaw plan, PTV V100 was reduced from 97.4% on the static planning CT to 86.2% and 88.8% on the averaged and time-weighted dose accumulations, respectively. A similar reduction in PTV V100 between the static calculation and accumulation was observed for the dynamic 2.5 cm jaw plan. Ipsilateral lung V20 was constant to within 2%.

Conclusion: We have observed a difference in PTV coverage between dose-volumes calculated on a single free-breathing CT image and a 4DCT after accumulation using a respiratory motion model for a TomoTherapy lung SBRT plan. Preliminary results suggest the value of a more comprehensive study simulating respiratory motion during delivery using a breathing motion model and the GPU-based TomoTherapy dose calculation engine.

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