Abstract
Although portal dosimetry is used to provide quality assurance (QA) for intensity‐modulated radiation therapy (IMRT) treatment plans, trends in agreement between the portal dose prediction (PDP) and the measured dose have not been clarified. In this work, we evaluated three scalar parameters of agreement for 152 treatment plans (1152 treatment fields): maximum gamma (γmax), average gamma (γavg), and percentage of the field area with a gamma value greater than 1.0 (γ%>1). These data were then used to set clinical action levels based on the institutional mean and standard deviations. We found that agreement between measured dose and PDP was improved by recalculating the fields at lower dose rates. We conclude that action levels are a useful tool for standardizing the evaluation of EPID‐based IMRT QA.PACS numbers: 87.53.Oq, 87.53.Mr, 87.53.Xd
Highlights
Different methods are used for quality assurance (QA) of intensity-modulated radiation therapy (IMRT) fields, including the use of film and ion chamber measurements and detector arrays, such as MapCHECK.[1,2,3,4] In our clinic, we recently implemented Varian Portal Dosimetry System (Varian Medical Systems, Palo Alto, CA), which uses an electronic portal imaging device (EPID), to measure the dose from the delivery of an IMRT treatment
To perform QA for IMRT with portal dosimetry, one uses analytic software to correlate the response of an amorphous silicon EPID to the dose delivered by applying position-specific factors from a dose calibration table
In an effort to provide action levels for physicists working in our facility, we evaluated our initial experience with IMRT QA with portal dosimetry
Summary
Different methods are used for quality assurance (QA) of intensity-modulated radiation therapy (IMRT) fields, including the use of film and ion chamber measurements and detector arrays, such as MapCHECK.[1,2,3,4] In our clinic, we recently implemented Varian Portal Dosimetry System (Varian Medical Systems, Palo Alto, CA), which uses an electronic portal imaging device (EPID), to measure the dose from the delivery of an IMRT treatment. To perform QA for IMRT with portal dosimetry, one uses analytic software to correlate the response of an amorphous silicon (aSi) EPID to the dose delivered by applying position-specific factors from a dose calibration table. This calibration table is determined using the calibration protocol outlined in the portal dosimetry system’s users’ manual. One must correlate the portal imager responses to the dose measured by a reference dosimeter, such as an ionization chamber with a calibration traceable to a National Institute of Standards and Technology (NIST) laboratory.
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