Abstract
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.
Highlights
The Electronic Portal Imaging Devices (EPID) is a very useful device for routine clinical use because of its prompt setup, easy data acquisition, and high resolution
The dosimetry process consists of three steps: (a) Fluence prediction using Eclipse Portal Dose Calculation (PDC) algorithm, (b) Fluence acquisition using dosimetry mode on the linac, and (c) Fluence comparison using the Portal Dosimetry module in Aria
Since the option of removing the flattening filter (FF) in the linacs for IMRT and VMAT treatments was introduced in 2010, there has been a lot of interest generated in using flattening filter-free (FFF) beams which give the benefit of reduced headscatter and reduced dose outside the field.[2]
Summary
The Electronic Portal Imaging Devices (EPID) is a very useful device for routine clinical use because of its prompt setup, easy data acquisition, and high resolution. Since the option of removing the flattening filter (FF) in the linacs for IMRT and VMAT treatments was introduced in 2010, there has been a lot of interest generated in using flattening filter-free (FFF) beams which give the benefit of reduced headscatter and reduced dose outside the field.[2]. These beams deliver dose faster than flattened beams, which would be beneficial for hypofractionated treatments by reducing treatment time and potential intrafractional organ motion.[3]
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