Program Information
Comprehensive Dose Verification of Single Isocenter Intracranial Radiosurgery Targets Located Distal From the Isocenter Using High Resolution 3D Dosimetry
M Trager1*, S Yoon1 , J Adamovics2 , M Oldham3 , J Adamson3 , (1) Duke University Medical Physics Graduate Program, Durham, NC, (2) Department of Chemistry and Biology, Rider University, Skillman, NJ, (3) Duke University Medical Center, Durham, NC
Presentations
TH-AB-FS1-5 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: Four Seasons 1
Purpose: Using a single-isocenter VMAT technique for radiosurgery of multiple targets is becoming more prevalent. In this study, we utilize a high-resolution 3D dosimetry system (PRESAGE®/Optical-CT) to validate the dosimetric accuracy specifically for targets located distal from the isocenter.Materials and
Methods: The multifocal radiosurgery treatment plan was prepared for a Varian STX linear accelerator with HD-MLCs. The plan had six targets (1cm diameter) located at varying distances (0-10cm) from the isocenter, each receiving 20Gy. A planning CT and SRS irradiation were carried out on two PRESAGE® dosimeters (1kg, 0cm diameter and height). Shifting the second dosimeter’s isocenter ensured the 10cm target was located away from the dosimeter periphery. 3D dose was recorded pre- and post-irradiation using a large FOV optical-CT (360 projections, 100 averages/projection, 174.3x174.3μm resolution/projection) with DLOS (Duke Large Optical Scanner). The optical-CT was de-noised using a 5x5 median filter, with a final reconstruction voxel resolution of 1x1x1mm. Dose (post-pre) was normalized to mean dose delivered to the target 3cm from the isocenter for both trials. Planned and delivered dose distributions were registered and analyzed using 3D Slicer software.
Results: Mean agreement per target using a 1mm/5% gamma analysis with a 30% dose threshold for 0, 3, 4, 6, 8, and 10cm targets was 78.0%, 81.7%, 78.9%, 85.9%, 85.7%, and 66.3%, respectively. Average ratio of measured to planned mean target dose was 1.06, 1.00±0.00, 0.97±0.01, 0.98±0.05, 0.97±0.04, and 0.93±0.01, respectively, while the average ratio of measured to planned D99% was 1.06, 1.02±0.04, 0.99±0.06, 0.98±0.06, 0.96±0.00, and 0.89±0.00, respectively.
Conclusion: Decreased agreement between measured and planned dose as well as an observed decrease in coverage suggest potentially less accurate dose calculation for targets located >8cm from the isocenter. Further investigation is warranted, which will include additional 3D dosimetry with varied geometry and verification using a second independent dosimetry system.
Funding Support, Disclosures, and Conflict of Interest: Dr. Adamson reports relationships with Immunolight LLC and Clearsight Radiotherapy, neither of which are related to this study.
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