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Two Beam Modulation Approaches Comparison: A Monte Carlo Study for MLC and Clinical Compensator Used for IMRT

John Zhai1,2*, (1) Simon Cancer Center, Radiation Oncology Department, Indiana University School of Medicine, Indianapolis, IN. (2) Tennessee Medical Physics, LLC, Nashville, TN


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Two important methods, multi-leaf collimators (MLC) and solid compensators, are used to do intensity modulated radiation therapy (IMRT) to produce an optimized primary fluence profile at the specific position of a patient through the middle of the tumor. A Monte Carlo (MC) study for characterization of solid brass compensators used for IMRT and comparison of two beam modulation approaches are performed to investigate their dosimetric properties.

Methods: We studied the beam perturbations caused by the presence of a solid brass compensator in radiation therapy field with Monte Carlo EGSnrc codes (BEAMnrc and DOSXYZnrc). A Varian 23iX linear accelerator was commissioned prior to the simulation of the compensators. The accelerator setup includes an electron source, a target slab, a primary tungsten conical shaped collimator, a Varian flatten filter, ion chamber, movable jaws, and a brass compensator. The simulation included five solid slab brass compensators and three clinical purposed brass compensators with various thicknesses and shapes. Phase-space files were scored before and after the compensators with MC codes BEAMnrc. The phase space files were then used as sources in DOSXYZnrc and dose was calculated in a solid 30x30x20cm3 water phantom at 100cm SSD for 6MV.

Results: Dosimetric data analysis of the PDDs and profiles showed that beam hardening does not have a significant effect on the PDD as it does on the energy spectrum. The PDDs are more penetrating than their open-field counterparts. Comparing results from MLC-based IMRT fields and compensators, we find them to be consistently dosimetrically equivalent for all the fields we studied.

Conclusion: MLC and compensators are consistently dosimetrically equivalent for all the fields we had studied. It shows that compensator is an inexpensive and reliable dose delivery device for IMRT and produces an optimized dose profile similar to the one obtained with MLC based IMRT delivery.


Monte Carlo, MLC, Compensators


TH- External beam- photons: Monte Carlo

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