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A Monte Carlo-Based Assessment of Scatter Contribution in Jaws Vs. MLCs Defined Small Fields for Varian Edge Linear Accelerator

K Xie*, E Salari , D Shvydka , A Qureshi , N Sperling , J Kunz , E Parsai , University of Toledo Medical Center, Toledo, OH


(Monday, 7/15/2019) 3:45 PM - 4:15 PM

Room: Exhibit Hall | Forum 3

Purpose: With a wide acceptance of high-dose SBRT and SRS approaches in radiation therapy the necessity of accurate dose computation for small fields becomes of crucial importance. Treatment planning system (TPS) commissioning requires the measured output factors (OFs) for small (<3x3cm2) fields to be loaded in the TPS. These OFs are typically measured for jaw-defined fields, and used for TPS beam model. In practice, MLC-only defined fields are often used by TPS with jaws tracking the MLC’s. We investigate the difference in scatter generated by the two configurations.

Methods: We built a Monte Carlo model of our clinical Varian Edge accelerator head using MCNP5 radiation transport package. Starting with the Varian provided phase-space file and limited geometry information, the 6FFF beam model was commissioned through detailed iterative comparison of the simulated and measured PDDs and profiles data. Next, output factors were modeled for jaws-only and MLCs-only defined fields for jaws settings from 0.6 to 3.0 cm at 10 cm depth. OFs were normalized to 5x5cm2 reference field and compared with those measured with Extradin W1 scintillator detector. Finally, the modeled data were analyzed to extract primary and scatter contributions.

Results: Model validation of our 6FFF beam presented challenges due to proprietary nature of the Edge head and inadequate spectral information provided. Through numerous iterative processes we were able to get reasonable agreement between computed and measured PDDs and profiles. At <10x10mm2 field sizes, there are approximately 18%, and 12% discrepancies between measured and modeled OF’s for jaws-only, and MLCs only defined configurations respectively. This discrepancy decreases as field sizes get larger to nearly unity at 3x3cm2.

Conclusion: Monte Carlo model provides a unique capability to dissect and analyze dosimetric data. Difference between jaws-only and MLCs-only defined configurations can be to a large extent explained by differences in scatter contributions.


MCNP, Small Fields


TH- External beam- photons: Small/nonstandard field Monte Carlo dosimetry

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