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A Comprehensive Geant4 Monte Carlo Model of Multileaf Collimator for Varian TrueBeam Linac

P Kandlakunta

P Kandlakunta*, T Zhang , S Mutic , R Khan , Washington University School of Medicine, St Louis, MO


SU-I-GPD-T-351 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To develop and validate a comprehensive Geant4 Monte Carlo model of Varian Millennium 120 multileaf collimator, thereby, provide an independent dose verification method for intensity modulated radiation therapy on a Varian Truebeamᵀᴹ Linac.

Methods: Monte Carlo model of Varian Truebeamᵀᴹ linac head was built using an in-house developed Geant4 application. Phase space data provided by Varian is supplied as the source particle input to Monte Carlo model. Modeling of upper and lower jaws and multileaf collimator (MLC) with adjustable settings is facilitated by use of macros in Geant4 application. Detailed MLC model was created including rounded leaf ends and tongue and grooves for each of 120 leaves in Geant4. As a first step, open field percent depth dose (PDD) and dose profiles at different depths were calculated in water phantom for the 6 MV beam for field sizes from 4×4 cm² to 40×40 cm². Both dose profile and PDD data are compared to measurements done on a Varian Truebeamᵀᴹ linac with a CC13 ionization chamber in water. As a next step, the MLC model will be evaluated by performing specific patterns to determine impact of inter-, intra- and leaf-end MLC transmission, tongue and groove and their impact on dosimetric penumbra and patient dose modeling in planning system.

Results: Comparison of open field 6 MV PDD and dose profiles with measured data indicated agreement better than 2% for all field sizes and at depths up to 10 cm. An agreement better than 4% was seen for profiles at depths greater than 20 cm.

Conclusion: At this preliminary stage, the Varian Truebeamᵀᴹ linac model was built in Geant4 and successfully validated with ionization chamber data from Truebeamᵀᴹ. A detailed Millennium 120-MLC geometry was represented accurately in simulations. Future work will entail simulation of impact of various MLC parameters on patient doses.

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