SU-E-T-688: Secondary MU Calculation for Flattening Filter Free (FFF) Photon Beams

Authors


Abstract

Purpose:

A secondary monitor unit (MU) checking algorithm was developed for flattening-filter free (FFF) photon beams. Conventional algorithms are based on the equivalent-square method to calculate the dose per MU. This is invalid because of the non-uniform profile of the FFF beam. An empirical point-spread kernel based algorithm that was previously developed was commissioned for a Varian TrueBeam FFF beam.

Methods:

Benchmark values at various off-axis points are checked for dose per MU and compared to the algorithm-calculated values for 6 and 10 MV FFF beams. Various SSD, jaw openings, and off-axis points were measured for each energy beam. The MU calculation program is then compared to Eclipse calculations for patient plans.

Results:

It was found that the developed MU calculating algorithm can be used as a secondary MU check for FFF photon beams for the TrueBeam. The calculated dose per MU were within 2% of the measured value for various beam settings. Varian Clinac benchmark values were also compared to the calculations.

Conclusion:

This work demonstrates the successful commissioning of a secondary MU calculation algorithm for photon FFF beams at 6 and 10 MV.

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