Program Information
Analytic Modelling of Elekta Infinity Linac: How Precise Model Parameters Need to Be for Small Radiation Fields?
S Gholampourkashi1*, J Cygler1,2 , J Belec2, M Vujicic2, E Heath1 , (1) Carleton Univ, Ottawa, ON, (2) The Ottawa Hospital Cancer Centre, Ottawa, ON
Presentations
TU-C3-GePD-J(B)-6 (Tuesday, August 1, 2017) 10:30 AM - 11:00 AM Room: Joint Imaging-Therapy ePoster Lounge - B
Purpose: To present an analytic model of the Elekta Infinity linac that helps to better understand the impact of input parameters on the isocenter photon fluence of small radiation fields.
Methods: An analytic photon beam model of an Elekta Infinity linac was benchmarked against Monte Carlo (MC) simulations. Parameters of the model including the MLC leaf bank rotation, angular and energy distributions were extracted from BEAMnrc simulations of the linac. Two virtual sources were used: 1) primary source to model the Bremsstrahlung photons from the target and 2) secondary source to model the scatter photons from the primary collimator and the flattening filter. The isocenter fluence due to a single open leaf (5×10 mm² at isocenter) was compared against MC calculations for the same field size to study the impact of model input parameters.
Results: Agreement of 1.1% was observed at the maximum fluence position between MC simulations and analytic calculations. Distance-to-Agreement (DTA) was found to be 0.02 mm at 50% of the maximum fluence. A fluence difference of 12% and DTA of 0.1 mm was observed if the leaf transmission effect is ignored. Using a uniform angular distribution rather than a precise model increased the fluence difference to 4% and the DTA to 0.05 mm. LBROT values of up to 12 mrad were found to increase the difference between MC and analytic model to as high as 8%. Increasing the primary source size from point source to 1.6 mm diameter was observed to increase this difference up to 14%.
Conclusion: Accurate modeling of input parameters for isocenter fluence calculations with analytic model is important when using small fields. More work is ongoing to explain the observed 14% dose difference between similar calculations with the Monaco treatment planning system and EBT3 film measurements.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Ontario Consortium of Adaptive Interventions in Radiation Oncology (OCAIRO), funded by the Ontario Research Fund Research Excellence program
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