Relative output factors of different collimation systems in truebeam STX medical linear accelerator

Purpose: Determine relative output factors of differrent collimation systems of a medical linear accelerator to ensure better modelling of the photon beam, especially small fields, as used in radiotherapy treatment planning systems; Method: The IAEA TRS483 and TRS398 Code of Practices (CoP) were used to calculate relative output factors for photon beams of 6X, 6XFFF energies with High Definition Multileaf Collimator (HDMLC), jaws and cones mounted on TrueBeam STx medical linear accelerator (Varian Medical Systems). A comparison between these results were made to show physicical characteristics of different collimation systems; Result: There is a large discrepancy in relative output factor curves found among different collimation systems of the same equivalent field sizes and between the CoPs. Conclusion: Specific beam modelling for each type of collimation system maybe required in TPS for better computation accuracy.
Keywords: TRS483 code of practice, small field dosimetry, relative output factors.