Program Information
Soft-Tissue / Bone Interface Backscatter Prediction by Two Commercial Electron Treatment Planning Algorithms
B Catt*, M Yudelev , McLaren - Macomb, Mt. Clemens, MI
Presentations
SU-E-T-707 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: In this work, we investigate the accuracy of two commercial Monte Carlo algorithms used for electron treatment planning in their prediction of dose enhancement near a soft-tissue / bone interface. The calculated doses at the interface of a heterogeneous phantom are compared with results obtained from measurements, simulations with EGSnrc, and calculations using an empirical formula.
Methods: Phantoms were constructed and modeled in Eclipse 10.0 (Varian) and Monaco 5.0 (Elekta) treatment planning systems. Slabs of bone equivalent material (CIRS Inc.) were inserted into a stack of solid water and situated at 2.9 cm depth, equivalent to dmax for a 12 MeV electron beam. This setup was chosen as an exaggeration of any clinically relevant inconsistencies which may be seen near interfaces. Plans were run on these phantoms and doses at the interface were compared with results obtained using a similar phantom within EGSnrc. The measurements were performed in a 12 MeV electron beam, using a parallel plate ion chamber.
Results: General agreement was observed between calculated and measured dose distributions along the beam’s central axis in the regions away from the heterogeneity; however, Varian’s eMC algorithm could not properly account for backscatter from higher density materials, instead predicting a decrease in dose just before the interface. This leads to an underestimation of dose by up to 9% at the interface, when compared with ion chamber measurements. Elekta’s VMC++ algorithm was able to more accurately account for the presence of the backscatter material, demonstrating an agreement within 1.7% of ion chamber measurements at the interface.
Conclusions: Although Monte Carlo algorithms are widely regarded as the gold standard for accuracy, care must still be taken when using implementations which have been optimized for calculation efficiency, especially within regions lacking electronic equilibrium such as those found near interfaces with bone.
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