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Modeling the Air Gap of Range Shifter for the Dose Calculation of IMPT

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Y Zhang

Y Zhang1*, M Kerr2 , J Hartman3 , W Qian1 , B Jiang2,4 , X Zhu2 , X Zhang2 (1) University of Texas, Houston, Texas, (2) The University of Texas MD Anderson Cancer Center, Houston, Texas, (3) University Medical Center Utrecht, Utrecht, Netherland, (4)Tianjin Medical University Cancer Institute and Hospital,Tianjin,China;


SU-I-GPD-T-131 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The dose calculation accuracy for spot scanning proton therapy with range shifter is lower than the average accuracy because the effect of the air gap between range shifter and patient on the depth dose was not modeled in analytical dose calculation algorithm. An improved dose calculation algorithm is developed to correctly modeling the air gap effect.

Methods: The Monte Carlo platform TOPAS was used to simulate the range shifter. The correction factors of the integrated depth dose for energy ranging from 72.5 to 221.8MeV with different air gaps are calculated. The measurement data are used to validate the correction factors for 219.3 MeV with 0, 10 cm, 30 cm air gap, 151.0 MeV and 181.3 MeV with 30 cm air gap. The gamma analysis was performed between calculation data with new model and measurement data for a head and neck case.

Results: A correction factor data set is built using Monte Carlo simulation. The calculated integral depth doses with different air gaps match the experiment data measured using Bragg peak chamber in a water tank. All errors within 5% for Energy from 151.0 MeV to 219.3 MeV and air gap from 10cm to 30 cm. The highest dose correction factor is 11.92% for 219.3 MeV with air gap 30cm with a mean error 0.2%, max error 0.7% compared with the measurement results. The head and neck plan pass rate is improved from 80.4% to 97.8% by applying the correction factor.

Conclusion: The inclusion of a correction factor for the air gap between range shifter and patient surface could significantly improve the dose calculation accuracy to meet the clinical accuracy.

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