Purpose: To implement a full Monte-Carlo beam source of the Varian TrueBeam head for primary standards establishment in terms of Dose Area Product (DAP) for 6 MV FFF small field sizes.
Methods: In Monte-Carlo dosimetry, multiple phase-spaces files from different Monte Carlo codes are available and free access. However, for small fields, the lack of detailed information and the reported limited accuracy are even more critical. As part of the establishment of the primary standards for small fields in DAP at the Laboratoire National Henri Becquerel (LNHB), the French Primary Standards Dosimetry Laboratory, and in order to overcome limits of Varian phase-space files, a full 6 MV FFF TrueBeam head has been modelled as a beam simulation source with the EGSnrc code with data provided by the manufacturer. A substantiated set of 11 configurations with specifically designed circular collimators of 5, 7.5, 10, 13 and 15 mm diameter and jaws of 5, 7, 10, 13, 15 and 20 mm side were investigated with respectively DOSXYZnrc and DOSRZnrc user codes. All the measurements were performed with a PTW 60019 Microdiamond and the validation was carried out with a strict gamma-index criteria of 0.5%/0.5mm, in accordance with metrological expectations.
Results: Experimental and Monte-Carlo calculations are found to be in excellent agreement. Commissioned Monte Carlo models reproduce the dose distributions of radiation field sizes from 5 mm to 20 mm with an average of 89 % and 98% of points passing the gamma-index criteria for respectively inline/crossline jaws profiles and percentage depth doses (PDD). For circular collimators, these values are 91 % and 96% for respectively radial profiles and PDD.
Conclusion: This robust validation of the 6 MV FFF source parameters allows a more confident use in the calculation of the graphite to water conversion factor, and possibly other correction factors.