Study on the impact of TRS-483 recommended correction factors for the dosimetry of flattening filter free small fields used in TrueBeam linear accelerator.

Purpose: To study the impact of TRS-483 recommended beam quality correction factor for reference dosimetry and field output correction factor for relative dosimetry of 6MV flattening filter free (FFF) small fields used in Varian Truebeam® linear accelerator (LINAC). Materials and Methods: Beam quality and Field output correction of 6MV FFF beam were adopted from TRS-483. Monte Carlo (MC) simulation of output factor was performed using PRIMO (PENELOPE based program) and compared with TRS-483 corrected output factors. Two Analytical Anisotropic Algorithm (AAA) models in EclipseTM treatment planning system (TPS) were created, one with output factor taken as ratio of meter readings and another model with output factor obtained by multiplying TRS-483 correction factor to the ratio of meter readings. Box field and Dynamic Conformal Arc (DCA) plans were created for both AAA models for verification and validation. Patient specific plan quality assurance (QA) was performed and deviation in point dose between measured and TPS calculated for both the models was analyzed. Results and Discussion: Separate beam quality correction factors provided for FFF in TRS-483 resulted in only 0.1% improvements for the reference dosimetry.TRS-483 corrected output factor are in better agreement with MC calculated output factor (OF) values. For the patient specific QA of DCA plans, output factor corrected AAA dose algorithm gave better agreement between measured and simulated dose. There is 1.2% less deviation observed for the smallest target of 0.23cc; 8mm equivalent sphere diameter used in this study. Conclusion: Field output factors for the LINAC small beams can be improved by incorporating the correction factors provided by TRS-483. But the extent of improvement that can be expected depends on the source model of calculating algorithm and how well generalized recommended corrections suit for user beam and the detector.