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




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.


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.


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.


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