Abstract
To present a clinical case in which real-time intratreatment imaging identified an multileaf collimator (MLC) leaf to be consistently deviating from its programmed and logged position by >1 mm. An EPID-based exit-fluence dosimetry system designed to prevent gross delivery errors was used to capture cine during treatment images. The author serendipitously visually identified a suspected MLC leaf displacement that was not otherwise detected. The leaf position as recorded on the EPID images was measured and log-files were analyzed for the treatment in question, the prior day's treatment, and for daily MLC test patterns acquired on those treatment days. Additional standard test patterns were used to quantify the leaf position. Whereas the log-file reported no difference between planned and recorded positions, image-based measurements showed the leaf to be 1.3 ± 0.1 mm medial from the planned position. This offset was confirmed with the test pattern irradiations. It has been clinically observed that log-file derived leaf positions can differ from their actual position by >1 mm, and therefore cannot be considered to be the actual leaf positions. This cautions the use of log-based methods for MLC or patient quality assurance without independent confirmation of log integrity. Frequent verification of MLC positions through independent means is a necessary precondition to trust log-file records. Intratreatment EPID imaging provides a method to capture departures from MLC planned positions.
Highlights
While AAPM TG-142 recommends qualitative weekly and quantitative monthly quality assurance (QA) of multileaf collimator (MLC) leaf positions,1 our clinic performs a daily QA leaf position check
The leaf position as recorded on the EPID images was measured and log-files were analyzed for the treatment in question, the prior day’s treatment, and for daily MLC test patterns acquired on those treatment days
A recent clinical observation hastened our upgrade to quantitative image-based daily MLC position QA and exposed a discrepancy which cautions the use of MLC log-files for routine MLC or patient-specific QA
Summary
While AAPM TG-142 recommends qualitative weekly and quantitative monthly quality assurance (QA) of multileaf collimator (MLC) leaf positions, our clinic performs a daily QA leaf position check. A recent clinical observation hastened our upgrade to quantitative image-based daily MLC position QA and exposed a discrepancy which cautions the use of MLC log-files for routine MLC or patient-specific QA. While agreement between logged and EPID measured leaf positions was typically found, position variations of up to 1.5 mm for test fields have been reported.. While agreement between logged and EPID measured leaf positions was typically found, position variations of up to 1.5 mm for test fields have been reported.2 Based on this observation, Agnew et al note that log-file based QA should not be solely relied upon as it does not always detect systematic machine faults.. We provide further support for this question via a clinical observation recorded by our real-time EPID-based treatment monitoring software. During a patient’s treatment, planned and logged leaf positions were in close agreement, the during treatment imaging revealed a 1.3 ± 0.1 mm difference (at the isocenter plane) in one leaf position
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