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Program Information

Volume Base Prescription for Electron Monte Carlo Monitor Unit Calculations

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J Baker

J Baker*, Y Valenciaga , E Klein , Northwell Health, Lake Success, NY


SU-I-GPD-T-62 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Monitor unit (MU) calculations using Monte Carlo code for electrons have statistical uncertainty resulting in MU variation for identical fields. The uncertainty can be reduced through simulating more particles, but this adds to the calculation time. Typically, a point based prescription is used for electrons and is effected by the uncertainty, but a volume based prescription averages the point uncertainties. Therefore, increased accuracy in MU calculation and reduction of calculation time may be obtained.

Methods: Utilizing Varian Eclipse v13.6 electron Monte Carlo calculation algorithm, eight fields were calculated using uncertainties of 2, 1, 0.7, 0.4 and 0.2%, with the smoothing function set to 3D Gaussian at medium level. The calculation time and MUs were determined for each field and uncertainty for a point based prescription, where 100% is the maximum dose on the central axis with the field prescribed to the 90% isodose line. V90% for the PTV was recorded for each field at the 0.2% uncertainty level. For the volume base prescription each field uncertainty level was then normalized so the PTV coverage was identical to the 0.2% uncertainty, and MU was recorded.

Results: The point base prescription yielded an average standard deviation in each field’s MUs across the different uncertainty levels at 0.15% of the total field MUs. A volume base prescription resulted in 0.05% variation in MU. When comparing the calculation times to the 2% uncertainty, the average increase in time for 1, 0.7, 0.4 and 0.2% uncertainty, are factors of 1.9, 3.1, 7.9 and 29.2 respectively.

Conclusion: The volume based prescription removed uncertainty from the MU calculation, though the point based was already accurate. The greatest benefit comes from nearly no loss in MU accuracy at the 2% level, which is not true for the point base prescription. This method yields significant time savings.

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