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Portal Dosimetry for SRS/SBRT Patient Specific Plan Quality Control

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R Zakikhani

R Zakikhani1*, H Lee2 , C Able3 , (1) Florida Cancer Specialists, Largo, FL, (2) Florida Cancer Specialists, The Villages, FL, (3) Florida Cancer Specialist, New Port Richey, FL


SU-I-GPD-T-547 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To determine an appropriate methodology for portal dosimetry (PD) analysis of SRS/SBRT deliveries to provide a pathway to transition from diode array (DA) analysis.

Methods: Our clinics have employed the use of both MapCheck2 and ArcCheck devices for stereotactic patient specific QC analysis for all delivery methods. Institutional passing criteria (gamma and absolute point dose) for the DA systems were established by statistical evaluation of thousands of QC results (>92%-SBRT, >93%-SRS). In transitioning to PD we need to determine how to define the region of interest (ROI) for analysis. Three approaches to defining the region of interest for gamma analysis with 2%-2mm were employed: 5% dose threshold (5-2-2); MLC complete irradiation area outline (CIAO) plus 0.5 cm (MLC+0.5cm-2-2); and MLC CIAO plus 1.0cm (MLC+1cm-2-2). Sixteen patient SBRT and/or SRS cases were analyzed using PD applying these ROI methods to every treatment field.

Results: The pass rates were lower for PD due to the higher detector resolution (0.7mm), thereby detecting more failures surrounding a single detector location than in the DA. The pass rates were lowest using the 5-2-2 criteria. Relative to the 5-2-2 criteria, the pass rates were +1.1% (MLC+0.5cm-2-2) and +3.0% (MLC+1cm-2-2). Statistical analysis showed that the passing criteria for PD, defined by the mean gamma score minus two standard deviations, was similar for IMRT and VMAT plans using the MLC ROI methods. The passing score for the PD MLC+0.5cm-2-2 ROI (92.0% overall) matched the DA criteria for SBRT. This ROI definition was selected as the initial methodology for our PD analysis of SRS/SBRT plans. Once additional QC results are gathered a reevaluation of passing criteria will be performed.

Conclusion: PD is an efficient clinical tool when compared to diode array systems. Institutional guidelines for the use of PD for SRS/SBRT patient specific plan QC were established.

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