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PHITS Monte Carlo Dose-Verification of VMAT Treatment Plan with HD120MLC

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N Nagaishi

N Nagaishi , F Araki , T Ohno , Kumamoto University, Kumamoto, Kumamoto, S Hashimoto , JAEA(Japan Atomic Energy Agency) ,Ibaraki


SU-I-GPD-T-358 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To verify volumetric modulated arc therapy (VMAT) plans with a high-definition multileaf collimator (HD20MLC) by comparing them with the dose distributions by the Particle and Heavy Ion Transport System (PHITS) Monte Carlo (MC) code.

Methods: The HD120MLC modeled by PHITS was verified by comparing dosimetric properties such as a leaf end transmission, inter/intra leaf transmission, and tongue and groove (T&G) effect with the EGSnrc/DOSxyznrc code and film measurements. In addition, the VMAT plan was created with the Acuros XB (AXB) in the Eclipse ver.10 treatment planning system (TPS) using the AAPM TG-119 prostate structure sets, and was recalculated by PHITS and EGSnrc. The VMAT dose distributions between three algorithms were compared by dose volume histograms (DVHs) and 3D gamma analysis with criteria of 2%/2mm and threshold of 10%.

Results: The maximum leaf end transmission for PHITS, EGSnrc, and film measurements was 21.2%, 21.2% and 21.8% of the maximum dose, respectively. Similarly, the average inter/intra leaf transmissions were 1.17%, 1.19%, and 1.08% respectively, and the dose reduction by the T&G effect in 2.5mm leaves was -25.5%, -25.8%, and -26.3%, respectively. The dosimetric properties of the HD120MLC calculated with PHITS were in good agreement with those of EGSnrc and film measurements. In dose verification for the VMAT plan, the DVHs of three algorithms agreed except for high dose region in PTV. The D2 of PTV of PHITS was 3.7% higher than EGSnrc and AXB. The passing rate of 3D gamma analysis between PHITS and AXB was more than 90% in PTV, rectum, and bladder.

Conclusion: The dosimetric properties of the HD120MLC modeled using PHITS was in good agreements with those of EGSnrc and film measurements. In dose verification for the VMAT plan, dose distributions calculated by PHITS was in almost agreement with those of EGSnrc and AXB.

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