Dataset Information

Small-field dosimetry of TrueBeamTM 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 (OFdet ) measured with different detectors were evaluated. To investigate the detector-associated effects, we evaluated the inter-unit variations of the OFdet 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 OFdet showed large variations exceeding 15% for a field size of 5 × 5 mm2 . When the output correction factors were applied to the OFdet , 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

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Publications

Small-field dosimetry of TrueBeam<sup>TM</sup> flattened and flattening filter-free beams: A multi-institutional analysis.

Akino Yuichi Y Mizuno Hirokazu H Isono Masaru M Tanaka Yoshihiro Y Masai Norihisa N Yamamoto Toshijiro T

Journal of applied clinical medical physics 20191209 1

<h4>Purpose</h4>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.<h4>Methods</h4>The beam data of 6- and 10-MV photon beams with and without flattening filter measured for modeling of an iPLAN treatment planning s ...[more]

PMID: 31816176

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Project description:As flattening filter-free (FFF) photon beams become readily available for treat-ment delivery in techniques such as SBRT, thorough investigation of skin dose from FFF photon beams is necessary under clinically relevant conditions. Using a parallel-plate PTW Markus chamber placed in a custom water-equivalent phantom, surface-dose measurements were taken at 2 × 2, 3 × 3, 4 × 4, 6 × 6, 8 × 8, 10 × 10, 20 × 20, and 30 × 30 cm2 field sizes, at 80, 90, and 100 cm source-to-surface distances (SSDs), and with fields defined by jaws and multileaf collimator (MLC) using multiple beam energies (6X, 6XFFF, 10X, and 10XFFF). The same set of measurements was repeated with the chamber at a reference depth of 10cm. Each surface measurement was normalized by its corresponding reference depth measurement for analysis. The FFF surface doses at 100 cm SSD were higher than flattened surface doses by 45% at 2 × 2 cm2 to 13% at 20 × 20 cm2 for 6MV energy. These surface dose differences varied to a greater degree as energy increased, ranging from +63% at 2 × 2 cm2 to -2% at 20 × 20 cm2 for 10 MV. At small field sizes, higher energy increased FFF surface dose relative to flattened surface dose; while at larger field sizes, relative FFF surface dose was higher for lower energies. At both energies investigated, decreasing SSD caused a decrease in the ratios of FFF-to-flattened surface dose. Variability with SSD of FFF-to-flattened surface dose differences increased with field size and ranged from 0% to 6%. The field size at which FFF and flattened beams gave the same skin dose increased with decreasing beam energy. Surface dose was higher with MLC fields compared to jaw fields under most conditions, with the difference reaching its maximum at a field size between 4 × 4 cm2 and 6 × 6 cm2 for a given energy and SSD. This study conveyed the magnitude of surface dose in a clinically meaning-ful manner by reporting results normalized to 10 cm depth dose instead of depth of dose maximum.

Dataset Information

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

Publications

Small-field dosimetry of TrueBeam<sup>TM</sup> flattened and flattening filter-free beams: A multi-institutional analysis.

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