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Dosimetric Validation of Single Isocenter VMAT for Multi-Lesion SRS/SRT Treatment

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N Neba

N Neba1*, N Demez2 , Y Altundal2 , (1) University of Kansas Cancer Center, Overland Park, KS, (2) University of Kansas Hospital, Kansas City, Kansas, (2) University of Kansas Hospital, Kansas City, KS


SU-I-GPD-T-619 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To dosimetrically verify the accuracy of the delivered dose using a single isocenter VMAT for multiple lesions SRS/SRT on a linac.

Methods: Four tumors were created at different depths in a water equivalent head phantom consisting of numerous slices using BBs to represent the tumors. The phantom was imaged on a Philips CT scanner with 1.5mm slice thickness. The tumors were contoured and expanded by 1mm to create the PTVs. OARs included: brain stem, eyes, and lenses. A single isocenter VMAT plan with two arcs was created in Eclipse TPS with a prescription dose of 20Gy to each lesion in one fraction and normalization of 100% dose covering 99% of PTV. The overall hot spot was 108%. The plan was QAed using ArchCheck. The BBs were replaced by mosfets, including one to the left eye for dose measurement. The phantom was “treated” on a Trilogy linac to mimic real patient treatment. A CBCT was taken with resulting shifts of 1mm vertically, 2mm longitudinally, and 1mm laterally.

Results: For arc 1, the IMRT QA results had 93.5% passing rate with 2% dose difference and 2mm DTA criteria, and 98.5% with 3%/3mm criteria. Arc 2 had 92.2% with 2%/2mm and 98.7% with 3%/3mm criteria. The mosfet results showed good agreement between the overall maximum doses and the measured doses; 0.75% difference for PTV1, 5.4% for PTV2, 1.75% for PTV3, 3.86% for PTV4, and 8.3% for left eye.

Conclusion: This work shows the potential for using a single isocenter VMAT plan to treat multiple SRS/SRT lesions. A very short treatment time of 23 minutes for single isocenter compared to 1.5–2 hours using multiple isocenters for four lesions was achieved. Shorter treatment times mean more patient comfort, less burden to staff and less room for errors.

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