A Monte Carlo approach to validation of FFF VMAT treatment plans for the TrueBeam linac

Journal Article · Fri Feb 15 00:00:00 EST 2013 · Medical Physics

DOI:https://doi.org/10.1118/1.4773883· OSTI ID:22130511

Gete, Ermias; Duzenli, Cheryl; Bergman, Alanah Mary ^[1]; Milette, Marie-Pierre; Mestrovic, Ante; Hyde, Derek; Teke, Tony ^[2]

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1. Medical Physics, BC Cancer Agency, Vancouver Centre, Vancouver, British Columbia V5Z 4E6 (Canada)
2. Medical Physics, BC Cancer Agency, Centre for Southern Interior, Kelowna, British Columbia V1Y 5L3 (Canada)

Purpose: To commission and benchmark a vendor-supplied (Varian Medical Systems) Monte Carlo phase-space data for the 6 MV flattening filter free (FFF) energy mode on a TrueBeam linear accelerator for the purpose of quality assurance of clinical volumetric modulated arc therapy (VMAT) treatment plans. A method for rendering the phase-space data compatible with BEAMnrc/DOSXYZnrc simulation software package is presented. Methods: Monte Carlo (MC) simulations were performed to benchmark the TrueBeam 6 MV FFF phase space data that have been released by the Varian MC Research team. The simulations to benchmark the phase space data were done in three steps. First, the original phase space which was created on a cylindrical surface was converted into a format that was compatible with BEAMnrc. Second, BEAMnrc was used to create field size specific phase spaces located underneath the jaws. Third, doses were calculated with DOSXYZnrc in a water phantom for fields ranging from 1 Multiplication-Sign 1 to 40 Multiplication-Sign 40 cm{sup 2}. Calculated percent depth doses (PDD), transverse profiles, and output factors were compared with measurements for all the fields simulated. After completing the benchmarking study, three stereotactic body radiotherapy (SBRT) VMAT plans created with the Eclipse treatment planning system (TPS) were calculated with Monte Carlo. Ion chamber and film measurements were also performed on these plans. 3D gamma analysis was used to compare Monte Carlo calculation with TPS calculations and with film measurement. Results: For the benchmarking study, MC calculated and measured values agreed within 1% and 1.5% for PDDs and in-field transverse profiles, respectively, for field sizes >1 Multiplication-Sign 1 cm{sup 2}. Agreements in the 80%-20% penumbra widths were better than 2 mm for all the fields that were compared. With the exception of the 1 Multiplication-Sign 1 cm{sup 2} field, the agreement between measured and calculated output factors was within 1%. It is of note that excellent agreement in output factors for all field sizes including highly asymmetric fields was achieved without accounting for backscatter into the beam monitor chamber. For the SBRT VMAT plans, the agreement between Monte Carlo and ion chamber point dose measurements was within 1%. Excellent agreement between Monte Carlo, treatment planning system and Gafchromic film dose distribution was observed with over 99% of the points in the high dose volume passing the 3%, 3 mm gamma test. Conclusions: The authors have presented a method for making the Varian IAEA compliant 6 MV FFF phase space file of the TrueBeam linac compatible with BEAMnrc/DOSXYZnrc. After benchmarking the modified phase space against measurement, they have demonstrated its potential for use in MC based quality assurance of complex delivery techniques.

• OSTI ID: 22130511
• Journal Information: Medical Physics, Vol. 40, Issue 2; Other Information: (c) 2013 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
62 RADIOLOGY AND NUCLEAR MEDICINE
60 APPLIED LIFE SCIENCES
COMPUTER CODES
DISTRIBUTION
DOSES
DOSIMETRY
FREE ENERGY
IONIZATION CHAMBERS
JAW
LINEAR ACCELERATORS
MONITORS
MONTE CARLO METHOD
PHASE SPACE
QUALITY ASSURANCE
RADIOTHERAPY
SIMULATION
SURFACES
THIN FILMS
VALIDATION