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End-To-End Evaluation of 4D-Dose Reconstruction Accuracy for Intra-Fractional Motion in Stereotactic Body Radiotherapy in High-Risk Prostate Cancer

N Hassan Rezaeian

N Hassan Rezaeian*, Y Chi , R Hannan , X Jia , The University of Texas Southwestern Medical Center, Dallas, TX


TH-EF-605-8 (Thursday, August 3, 2017) 1:00 PM - 3:00 PM Room: 605

Purpose: At our institution, a clinical trial on stereotactic body radiotherapy (SBRT) for high-risk prostate cancer is ongoing. It aims at dose escalation to intra-prostatic lesions. Intra-fractional prostate motion deteriorates well planned radiation dose. We have previously developed a GPU-based Monte-Carlo (MC) 4D-dose reconstruction system. In-treatment motion reconstruction is done by using kV-projections acquired during delivery and 4D-dose calculation is performed by using MC dose engine and trajectory log files. The purpose of this work is to perform an end-to-end evaluation on the accuracy of the system via a phantom study.

Methods: We constructed a prostate phantom using a water-equivalent material via 3D-printing. The phantom holds a film to measure planar dose in a plane containing prostate PTV, lesion, and urethra. It also contained an OSLD to measure point dose at urethra location. The phantom was driven by a motion stage to follow a realistic prostate motion trajectory. An SBRT plan using VMAT was generated to cover the prostate PTV to 9.5Gy and lesion to 10Gy. We first delivered the plan to the phantom without motion and compared measured and reconstructed doses to test MC dose calculation accuracy of our system. We then delivered the plan to a moving phantom as an end-to-end test of the overall accuracy.

Results: In the case without motion, measured point dose agreed with that from the treatment planning system (TPS) and from our system with differences<%1.5 and<%1.3. Gamma passing rates (2mm/2%) were 96.3% and 97.2% with TPS and our system, respectively. In the case with motion, measured point dose at urethra position was 4% higher than planned dose, indicating the impact of prostate motion. 4D-reconstructed dose agreed with the measured dose with<%2 difference for point dose and gamma passing rate 94.8%.

Conclusion: Comprehensive end-to-end tests validated the accuracy of our 4D-dose reconstruction system.

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