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Comparison of Methods to Convert Treatment Plans On Millennium MLC to HD120 MLC

G Shukla1*, E Meekins2 , (1) Geisinger Medical Center, Danville, PA, (2) Geisinger Health System, Danville, PA


(Tuesday, 7/16/2019) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 1

Purpose: To develop and compare different methods for converting VMAT plans in Eclipse for a Trilogy linear accelerator with Millennium MLC to a TrueBeam with HD120 MLC.

Methods: Patients planned on a Trilogy linac were converted to plans on a Truebeam using three methods-(A) Custom Eclipse API script to recreate MLC apertures and control points for new MLC and machine using same arc geometry as original plan.(B) Re-optimization with new MLC using same optimization objectives as original plan, and (C)Eclipse built-in DVH-based Plan Converter using 9 IMRT fields. Conformality Index (CI), Gradient Index (GI) and Max dose to target along with Maximum and Mean dose to four Organs at Risk (OAR) were recorded for eight patients before and after conversion using the three methods. All plans were normalized to 95% target coverage at the prescription dose. IMRT QA was performed to confirm original and script generated plans were both deliverable and comparable.

Results: The Script method(A) had .8% worse CI, reoptimization(B) was 0.1% worse while DVH-based plan converter(C) was 18.5% worse. (A) reduced maximum OAR dose by 0.2% while (B) increased it by .7% and (C)by 2.3% respectively. (A) and (B) reduced mean OAR dose by 0.7% and 1.5% respectively while the DVH plan converter increased it by 5.1%.

Conclusion: The custom aperture-based script and full VMAT re-optimization both yielded high quality plans while Eclipse’s DVH-based Plan Converter resulted in a significant loss of conformality and higher OAR doses. Since the plan conversion will likely be a time-sensitive task due to machine downtime, the script-based method may be preferable due to its speed(<30s). Additionally, the script based method can be used with plans imported from elsewhere and plans that don’t have prior optimization objectives saved in Eclipse.


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