Purpose Good clinical practice in small field dosimetry relies on using multiple detectors for the determination of relative output factors (ROFs). The TRS-483 Code of Practice (CoP) provides recommended output correction factors, k, for a range of detectors and beam energies as used in small field dosimetry. In this work, the convergence of the ROFs for 6MV x-ray beams with and without flattening-filters were investigated under a different combinations of beam collimation and published detector correction factors.
Method Three IBA detectors SFD, PFD and CC04 were used to measure ROFs on a TrueBeam STx linear accelerator with small fields collimated by the high definition MLC which has 2.5 mm and 5.0 mm wide leaves. Two configurations were used for the collimators being 1) fixed-jaws at 10 × 10 cm2 and 2) with a 2 mm offset from the MLC edge, in line with the recommended geometry from IROC-H as part of their auditing program and published dataset. The k factors for the three detectors were taken from the TRS483 CoP and other published works.
Results The standard deviation in measured ROFs between detectors under 6MV-WFF beam are 1.4% and 1.9% while these under 6MV-FFF beam are 2.3% and 2.4% in MLC field with fixed-jaw and with 2 mm jaws-offset, respectively. The relative difference in averaged ROFs of three reference detectors from corresponding IROC-H dataset are 2.0% and 3.1% under 6MV-WFF beam while they are 2.4% and 3.2% under 6MV-FFF beam at the smallest available field size 2 × 2 cm2. For smaller field sizes, the uncertainty in averaged ROFs of three reference detectors from corresponding results of from 2 mm jaws-offset setting by Akino and Dufreneix showed the largest difference are 6.6% and 6.2% under 6MV-WFF beam while they are 3.4% and 3.6% under 6MVWFF beam at smallest field size 0.5 × 0.5 cm2.
Conclusion The corresponding datasets provided by TRS-483 and IROC-H are generic and may not statistically large enough with 6MV-FFF and also MLC field with 2 mm jaws-offset setting, so that clinical users should use at least three different types of small field detector in measurements. The IROC-H dataset is not available for field size smaller than 2 × 2 cm2 so that user should carefully check with other publications with the same setting.