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3D Verification for Multifocal Single Isocenter SRS

A Thomas

A Thomas1*, M Niebanck2, Z Wang1, J Kirkpatrick1, M Oldham1, (1) Duke University Medical Center, Durham, NC, (2) Advanced Radiation Centers of New York, Hartdale, NY

MO-F-108-11 Monday 4:30PM - 6:00PM Room: 108

Purpose: Recent trends in SRS use multifocal VMAT plans to simultaneously treat several, distinct targets. Conventional verification often involves low resolution measurements in a single plane, cylinder, or intersecting planes of diodes or ion chambers. This work presents an investigation into the accuracy, consistency and reproducibility of this treatment technique using a high-resolution 3D dosimetry system (PRESAGE/Optical-CT).
Methods: A complex VMAT plan consisting of a single isocenter but 5 separate targets was created in Eclipse for a head phantom containing a cylindrical PRESAGE dosimetry insert of 10cm diameter and height. The plan contained 5 arcs delivering doses from 12-20 Gy. The treatment was delivered to four separate dosimeters positioned in the head-phantom. Each delivery was performed after IGRT positioning using 2D and 3D kilovoltage images. A final delivery was given to a modified insert containing a pin-point ion chamber enabling calibration of Presage data to dose. Dosimetric data was read out in an optical-CT scanner. Consistency and reproducibility of the treatment technique (including IGRT set-up) was investigated by comparing the dose distributions in the 4 inserts, and with the predicted TPS distribution.
Results: Dose distributions from the 4 dosimeters were registered to find the mean and standard deviation at all points throughout the dosimeters. They showed less than 3% standard deviation within voxels of similar dose regions. 3D gamma maps show the calculated delivered dose matched output to within expected tolerances and errors - passing rate was 98.0% (3%, 2mm).
Conclusions: The deliveries of the irradiation were found to be consistent and matched the treatment plan, demonstrating high accuracy and reproducibility of the treatment machine, the IGRT procedure, and the 3D measurement. The complexity of the treatment (multiple arcs) and dosimetry (multiple strong gradients) pose a substantial challenge for comprehensive verification. 3D dosimetry can be uniquely effective in this scenario.

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