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Monte Carlo Simulations for Quality Assurance of Varian TrueBeam 10XFFF VMAT SABR Treatments

E Gete

E Gete1*, A Bergman1, C Duzenli1,2, R Lee1, F Viel2, T Teke3, (1) BC Cancer Agency Vancouver Centre, Vancouver, BC, (2) University of British Columbia, Vancouver, BC, (3) BC Cancer Agency Centre for Southern Interior, Kelowna, BC

SU-E-T-411 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To establish feasibility of performing quality assurance on 10X Flattening Filter Free (FFF) VMAT stereotactic ablative radiotherapy (SABR) treatments on the TrueBeam LINAC with Monte Carlo simulations using vendor-supplied phase space data.
Monte Carlo simulations were performed with BEAMnrc and DOSXYZnrc using the TrueBeam 10XFFF photon beam phase space data that were made available by the Varian Monte Carlo research team. To establish validity of the phase space, dose calculations in a water phantom for fields ranging from 2x2 cm² to 40x40 cm² were performed using DOSXYZnrc. Percent depth doses (PDDs), transverse profiles and output factors were calculated and compared with measurements for all the fields simulated. Monte Carlo simulations of 10XFFF SABR VMAT plans were performed on a homogeneous cylindrical phantom and on the Quasar™ phantom with a lung-equivalent insert. Simulations were compared with both ion chamber measurement and Eclipse Treatment Planning System (TPS) dose calculations. 3D gamma analysis comparing Monte Carlo and TPS results was performed.
Monte Carlo simulations and measured values agreed within 1% and 2% for PDDs and profiles respectively. The agreement between measured and calculated output factors was within 1% including for highly asymmetric fields. These results indicate that the 10XFFF phase space data is sufficiently accurate for use in simulations for quality assurance purposes in radiation therapy. For the VMAT plans the point dose agreement between MC and both measured and TPS calculations was within 1.2%. The 3%, 3mm Gamma test pass rates were 95% and 92% for plans calculated on the homogeneous phantom and on the Quasar phantom respectively.
We have demonstrated the feasibility of performing patient specific quality assurance on 10XFFF SABR treatment plans using Monte Carlo simulations of the TrueBeam LINAC. This is the first independent validation on the 10XFFF Varian phase space data presented to date.

Funding Support, Disclosures, and Conflict of Interest: The work of Tony Teke, Cheryl Duzenli and Ermais Gete has been supported by Varian under a Master Research Agreement.

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