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(Sunday, 7/29/2018) 2:05 PM - 3:00 PM
Room: Karl Dean Ballroom A2
Purpose: The Varian Halcyon™ portal imager automatically acquires exit dosimetry images for every patient with full field coverage. This could facilitate “every patient, every monitor unit” QA and adaptive radiotherapy. This study evaluated the system’s sensitivity to potential clinical errors (beam output, patient weight, and misalignment changes) and examined day-to-day variations from clinical exit images.
Methods: Open 10x10 and modulated IMRT fields were delivered for each test. To evaluate output changes, monitor units were scaled by 2-10% and delivered to a solid water stack and CIRS anthropomorphic phantom. To mimic weight changes, 0.5-5.0cm of buildup were added to the solid water. To evaluate misalignment, the oval-shaped CIRS phantom was shifted laterally 2-10cm. For each test, the mean relative differences (MRD) and standard deviation of pixel-difference histograms (SD) between resulting and baseline images were calculated. To assess actual patient variations, MRD and SD were calculated from a clinical prostate patient’s exit images.
Results: The MRD responded linearly to output and buildup changes with shot-to-shot MRD variations of 0.4%, implying 1% output and 0.2cm changes in buildup could be detected with 2.5σ confidence if these were the only source of error. Shifting the oval-shaped CIRS phantom laterally resulted in detectable MRD changes, but SD increased at a higher rate suggesting it would be more sensitive to positioning offsets than MRD. Day-to-day variations in the MRD for a prostate patient ranged from -1.6% to 2.4%, meaning these variations are much larger than the sensitivity in any one type of error.
Conclusion: Rapid analyses of automatically-acquired Halcyon exit images could detect mid-treatment changes with high sensitivity, though appropriate thresholds for typical error types would necessitate more detailed pixel difference image analysis and/or multiple field observations. This study presents the first steps towards developing effortless image evaluation for all aspects of every patient’s treatment.
Funding Support, Disclosures, and Conflict of Interest: Conflicts of interest: Varian research grant, travel, and honoraria.
Keywords
Electronic Portal Imaging, Transit Dosimetry, Quality Assurance
Taxonomy
TH- External beam- photons: portal dosimetry
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