Small-field dosimetry of TrueBeam^TM flattened and flattening filter-free beams: A multi-institutional analysis.

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 mm² 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 mm² field size were 0.97 mm. The OF_det showed large variations exceeding 15% for a field size of 5 × 5 mm² . 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 mm² , and most occurred in a detector-dependent manner.

SUBMITTER: Akino Y 

PROVIDER: S-EPMC6964782 | biostudies-literature | 2020 Jan

REPOSITORIES: biostudies-literature