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Commissioning a Monte Carlo Dose Calculation Algorithm in Heterogeneous Media for a Magnetic Resonance Image Guided Linear Accelerator

A Price*, A Curcuru , F Reynoso , J Cammin , S Mutic , O Green , Washington University School of Medicine, St. Louis, MO


(Monday, 7/30/2018) 4:30 PM - 5:30 PM

Room: Exhibit Hall | Forum 3

Purpose: This work focused on commissioning the Monte Carlo dose calculation algorithm for a poly-energetic source emitted from a magnetic resonance image (MRI) guided linear accelerator (Linac) using heterogeneous phantoms. The poly-energetic source will create secondary electrons with various energies that will experience a Lorentz force which can have a substantial effect in lower density media due to larger path lengths.

Methods: The MR-Linac utilizes a 6MV flattening filter free beam with a double stacked and double focused multi-leaf collimator (MLC) under the influence of a 0.35T MR scanner. As a part of commissioning, to test the treatment planning system’s (TPS) ability to model and calculate dose using a Monte Carlo dose calculation algorithm, measurements were made in an in-house heterogeneity phantom consisting of a water annulus containing a polystyrene lesion within cork. The phantom allows for 1D ion chamber and 2D radiochromic film measurements, testing dose buildup, electron return effect, and heterogeneity dose calculations. In addition to the heterogeneity phantom, a commercial 3D dose measurement device with heterogeneous inserts and an ion chamber insert was used to assess the TPS’s dose calculation algorithm within the magnetic field. A suite of simple field geometries and intensity-modulated radiation therapy (IMRT) plans were used in the delivery.

Results: For the heterogeneity phantom, the average ratio of ion chamber readings to expected readings is 1.006±0.013. The film measurements average gamma index was 94.74%±3.78%. For the 3D dose measurements, the average gamma index between the measurement and what was calculated is 98.45%±0.25%. The average ion chamber ratio for the 3D dose measurement was 1.020.

Conclusion: Our study demonstrates an extensive and thorough set of measurements for commissioning a novel commercial MR-Linac TPS Monte Carlo dose calculation algorithm. The TPS successfully models dose based on measurements made utilizing multiple measurement techniques in heterogeneous phantoms.


Not Applicable / None Entered.


TH- External beam- photons: dose computation engines- Monte Carlo

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