Abstract
PURPOSE: Detector-dependent interinstitutional variations of the beam data may lead to uncertainties of the delivered dose to patients. Here we evaluated the inter-unit variability of the flattened and flattening filter-free (FFF) beam data of multiple TrueBeam (Varian Medical Systems) linear accelerators focusing on the small-field dosimetry. METHODS: The beam data of 6- and 10-MV photon beams with and without flattening filter measured for modeling of an iPLAN treatment planning system (BrainLAB) were collected from 12 institutions - ten HD120 Multileaf Collimator (MLC) and two Millennium120 MLC. Percent-depth dose (PDD), off-center ratio (OCR), and detector output factors (OF(det) ) measured with different detectors were evaluated. To investigate the detector-associated effects, we evaluated the inter-unit variations of the OF(det) before and after having applied the output correction factors provided by the International Atomic Energy Agency (IAEA) Technical Reports Series no. 483. RESULTS: PDD measured with a field size of 5 × 5 mm(2) showed that the data measured using an ionization chamber had variations exceeding 1% from the median values. The maximum difference from median value was 2.87% for 10 MV photon beam. The maximum variations of the penumbra width for OCR with 10 × 10 mm(2) field size were 0.97 mm. The OF(det) showed large variations exceeding 15% for a field size of 5 × 5 mm(2) . When the output correction factors were applied to the OF(det) , the variations were greatly reduced. The relative difference of almost all field output factors were within ± 5% from the median field output factors. CONCLUSION: In this study, the inter-unit variability of small-field dosimetry was evaluated for TrueBeam linear accelerators. The variations were large at a field size of 5 × 5 mm(2) , and most occurred in a detector-dependent manner.