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Single Target Radiosurgery Quality Assurance with Portal Dosimetry

E Covington

E Covington*, J Snyder , X Wu , R Popple , Univ Alabama Birmingham, Birmingham, AL


SU-I-GPD-T-256 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To assess the feasibility of using portal dosimetry (PD) for pre-treatment quality assurance of single target, flattening filter free (FFF), volumetric arc therapy intracranial radiosurgery plans.

Methods: A PD algorithm was created for a 10X FFF beam on a Varian Edge linear accelerator (Varian Inc, Palo Alto, CA). Treatment plans that were previously evaluated with Gafchromic EBT-XD (Ashland, Bridgewater, NJ) film were measured via PD and analyzed with the ARIA Portal Dosimetry workspace. Film analysis was done by normalizing to the treatment planning system (TPS) mean dose in the region receiving greater than or equal to 90% of the max dose. After applying this normalization factor to the film, a 3%/1 mm gamma analysis was performed. This process was recreated in the PD workspace by creating a script to determine the normalization factor, applying it to the portal image, and then performing a 3%/1 mm gamma analysis.

Results: Thirty-eight PD verification plans were delivered and analyzed. The average PD to TPS normalization was 0.992 ± 0.02 while film to TPS normalization was 1.025 ± 0.01. After applying the normalization factor, all PD plans passed the gamma analysis with a 10% dose threshold with 97% of points having gamma < 1. Overall, PD to TPS normalization was found to be target size dependent. As target size decreases, the normalization factor decreased from 1.004 for targets with diameters between 15-44 mm and 0.985 for targets with diameters than 15 mm.

Conclusion: The agreement of PD to TPS mean dose in the high dose region was found to be dependent on target size. Film measurements did not exhibit size dependence. All PD plans passed the 3%/1 mm gamma analysis after normalization, but caution should be used when using PD to assess overall dosimetric accuracy of the treatment plan for small targets.

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