Monte Carlo and analytic modeling of an Elekta Infinity linac with Agility MLC: Investigating the significance of accurate model parameters for small radiation fields.

Sara Gholampourkashi,Jason Belec,Miro Vujicic,Emily Heath,Joanna E Cygler

doi:10.1002/acm2.12485

Sara Gholampourkashi, Jason Belec + Show 3 more

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https://doi.org/10.1002/acm2.12485

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Abstract

PurposeTo explain the deviation observed between measured and Monaco calculated dose profiles for a small field (i.e., alternating open‐closed MLC pattern). A Monte Carlo (MC) model of an Elekta Infinity linac with Agility MLC was created and validated against measurements. In addition, an analytic model which predicts the fluence at the isocenter plane was used to study the impact of multiple beam parameters on the accuracy of dose calculations for small fields.MethodsA detailed MC model of a 6 MV Elekta Infinity linac with Agility MLC was created in EGSnrc/BEAMnrc and validated against measurements. An analytic model using primary and secondary virtual photon sources was created and benchmarked against the MC simulations and the impact of multiple beam parameters on the accuracy of the model for a small field was investigated. Both models were used to explain discrepancies observed between measured/EGSnrc simulated and Monaco calculated dose profiles for alternating open‐closed MLC leaves.Results MC‐simulated dose profiles (PDDs, cross‐ and in‐line profiles, etc.) were found to be in very good agreements with measurements. The best fit for the leaf bank rotation was found to be 9 mrad to model the defocusing of Agility MLC. Moreover, a very good agreement was observed between results from the analytic model and MC simulations for a small field. Modifying the radial size of the incident electron beam in the BEAMnrc model improved the agreement between Monaco and EGSnrc calculated dose profiles by approximately 16% and 30% in the position of maxima and minima, respectively.ConclusionAccurate modeling of the full‐width‐half‐maximum (FWHM) of the primary photon source as well as the MLC leaf design (leaf bank rotation, etc.) is essential for accurate calculations of dose delivered by small radiation fields when using virtual source or MC models of the beam.

Highlights

Monte Carlo (MC) techniques are accepted to be the most accurate method of dose calculation in radiotherapy and a reliable tool for modeling linear accelerators.[1]
Proper values for parameters in the MC model of a linear accelerator head play an important role in accurate dose calculations
We demonstrated the detailed MC modeling of a 6 MV Elekta Infinity linac with Agility MLC leaves that was validated against measurements

Summary

Introduction

Monte Carlo (MC) techniques are accepted to be the most accurate method of dose calculation in radiotherapy and a reliable tool for modeling linear accelerators (linacs).[1]. The sensitivity of the linac model to different parameters has been investigated by several groups.[5,6,9,11,13,14,15] Sheikh‐Bagheri and Rogers[5,6] studied beam parameters of nine megavoltage photon beams from different manufacturers (Varian, Elekta and Siemens) and concluded that MC simulations of photon beams are highly sensitive to the radial intensity and mean energy of the incident electron beam They reported that the accuracy of simulations is sensitive to the primary collimator opening and flattening filter material and density. This is the shape adopted in all studies that model beam parameters in MC simulations

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