Definition of parameters for quality assurance of flattening filter free (FFF) photon beams in radiation therapy

We proposed to perform a basic dosimetry commissioning on a new imager system, the Varian aS1200 electronic portal imaging device (EPID) and TrueBeam 2.0 linear accelerator for flattened (FF) and flattening filter‐free (FFF) beams, then to develop an image‐based quality assurance (QA) model for verification of the system delivery accuracy for intensity‐modulated radiation therapy (IMRT) treatments. For dosimetry testing, linearity of dose response with MU, imager lag, and effectiveness of backscatter shielding were investigated. Then, an image‐based model was developed to convert images to planar dose onto a virtual water phantom. The model parameters were identified using energy fluence of the Acuros treatment planning system (TPS) and, reference dose profiles and output factors measured at depths of 5, 10, 15, and 20 cm in water phantom for square fields. To validate the model, its calculated dose was compared to measured dose from MapCHECK 2 diode arrays for 36 IMRT fields at 10 cm depth delivered with 6X, 6XFFF, 10X, and 10XFFF energies. An in‐house gamma function was used to compare planar doses pixel‐by‐pixel. Finally, the method was applied to the same IMRT fields to verify their pretreatment delivery dose compared with Eclipse TPS dose. For the EPID commissioning, dose linearity was within 0.4% above 5 MU and ∼1% above 2 MU, measured lag was smaller than the previous EPIDs, and profile symmetry was improved. The model was validated with mean gamma pass rates (standard deviation) of 99.0% (0.4%), 99.5% (0.6%), 99.3% (0.4%), and 98.0% (0.8%) at 3%/3 mm for respectively 6X, 6XFFF, 10X, and 10XFFF beams. Using the same comparison criteria, the beam deliveries were verified with mean pass rates of 100% (0.0%), 99.6% (0.3%), 99.9% (0.1%), and 98.7% (1.4%). Improvements were observed in dosimetric response of the aS1200 imager compared to previous EPID models, and the model was successfully developed for the new system and delivery energies of 6 and 10 MV, FF, and FFF modes.PACS number(s): 87.53.Oq, 87.53.Xd

Section: Introductionmentioning

confidence: 99%

EPID‐based dosimetry to verify IMRT planar dose distribution for the aS1200 EPID and FFF beams

Miri

Keller

Zwan

et al. 2016

“…Pönisch et al (2) and Fogliata et al (13) proposed different methods to normalize the FFF profile by using the inflection point and shoulder point, respectively. We used the normalization method suggested by Fogliata and colleagues to compare the field widths, penumbras, and doses in the lateral regions of the profiles between flattened and FFF beams.…”

Section: Methodsmentioning

confidence: 99%

“…However, when the FFF beam is normalized to the same dose level as the flattened beam, as suggested by Fogliata et al, (13) it is possible to evaluate the penumbra as the difference in position between the 80% and 20% dose levels of the profile.…”

Section: Methodsmentioning

confidence: 99%

“…However, when the FFF profile is renormalized, as previously described, a region within a certain percentage of the field can be used to define a parameter for evaluating the off‐axis profile variation for both beam modalities 6 , 10 , 13 , 15 . The off‐axis profile variations of both flattened and FFF beams for different field sizes at changing depths were quantified by using the off‐axis ratio (OAR).…”

Section: Methodsmentioning

confidence: 99%

“…Kragl et al (6) studied both 6 MV and 10 MV unmatched FFF beams and also investigated only the 6 MV matched FFF beam from an Elekta Precise linac. In this paper, we will follow the designs of the previous studies, 1 , 2 , 3 , 6 , 7 , 8 , 9 , 10 , 11 , 13 , 14 particularly the Huang et al (10) study on the quality matched 6MVFFF beam provided by a Siemens linac, to provide medical physics community with our evaluation on the dosimetric differences between conventional flattened and equivalent‐quality FFF beams of lower and higher energies from Elekta Versa HD linac.…”

Section: Introductionmentioning

confidence: 99%

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Dosimetric properties of equivalent‐quality flattening filter‐free (FFF) and flattened photon beams of Versa HD linear accelerator

Meshram

Pramanik

Ranjith

et al. 2016

This study presents the basic dosimetric properties of photon beams of a Versa HD linear accelerator (linac), which is capable of delivering flattening filter‐free (FFF) beams with a beam quality equivalent to the corresponding flattened beams based on comprehensive beam data measurement. The analyzed data included the PDDs, profiles, penumbra, out‐of‐field doses, surface doses, output factors, head and phantom scatter factors, and MLC transmissions for both FFF and flattened beams of 6 MV and 10 MV energy from an Elekta Versa HD linac. The 6MVFFF and 10MVFFF beams had an equivalent mean energy to the flattened beams and showed less PDD variations with the field sizes. Compared with their corresponding flattened beams, Dmax was deeper for FFF beams for all field sizes; the ionization ratio variations with the field size were lower for FFF beams; the out‐of‐field doses were lower and the penumbras were sharper for the FFF beams; the off‐axis profile variations with the depths were lesser for the FFF beams. Further, the 6MVFFF and 10MVFFF beams had 35.7% and 40.9% less variations in output factor with the field size, respectively. The collimator exchange effect was reduced in the FFF mode. The head scatter factor showed 59.1% and 73.6% less variations, on average, for the 6MVFFF and 10MVFFF beams, respectively; the variations in the phantom scatter factor were also smaller. The surface doses for all beams increased linearly with the field size. The 6MVFFF and 10MVFFF beams had higher surface doses than the corresponding flattened beams for field sizes of up to 10×10cm2 but had lower surface doses for larger fields. Both FFF beams had lower average MLC transmissions than the flattened beams. The finding that the FFF beams were of equivalent quality to the corresponding flattened beams indicates a significant difference from the data on unmatched FFF beams.PACS number(s): 87.56.bd, 87.55.Qr

Dose rate response of Digital Megavolt Imager detector for flattening filter‐free beams

Kim

Han

et al. 2018

In this study we investigated the dose rate response characteristics of the Digital Megavolt Imager (DMI) detector, including panel saturation, linearity, and imager ghosting effects for flattening filter‐free (FFF) beams. The DMI detector dose rate response characteristics were measured as a function of dose rate on a Varian TrueBeam machine. Images were acquired at dose rates ranging from 400 to 1400 MU/min for 6XFFF and 400 to 2400 MU/min for 10XFFF. Line profiles and central portal doses derived from the images were analyzed and compared. The linearity was verified by acquiring images with incremental Monitor Unit (MU) ranging from 5 to 500 MU. Ghosting effects were studied at different dose rates. Finally, for validation, test plans with optimal fluence were created and measured with different dose rates. All test plans were analyzed with a Gamma criteria of 3%‐3 mm and 10% dose threshold. Our study showed that there was no panel saturation observed from the profile comparison even at the maximum dose rate of 2400 MU/min. The central portal doses showed a slight decrease (1.013–1.008 cGy/MU for 6XFFF, and 1.020–1.009 cGy/MU for 10XFFF) when dose rate increased (400–1400 MU/min for 6XFFF, and 400–2400 MU/min for 10XFFF). The linearity of the DMI detector response was better than 0.5% in the range of 20–500 MU for all energies. The residual image was extremely small and statistically undetectable. The Gamma index measured with the test plans decreased from 100% to 97.8% for 6XFFF when dose rate increased from 400 to 1400 MU/min. For 10XFFF, the Gamma index decreased from 99.9% to 91.5% when dose rate increased from 400 to 2400 MU/min. We concluded that the Portal Dosimetry system for the TrueBeam using DMI detector can be reliably used for IMRT and VMAT QA for FFF energies.