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A New 3D Gamma Analysis Algorithm for Better VMAT/IMRT QA Results

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D Cao

D Cao*, J Ye , D Shepard , Swedish Cancer Institute, Seattle, WA


SU-E-T-468 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:The gamma dose distribution comparison method is used in VMAT QA to score the agreement between planned and measured dose distributions. Unfortunately, this dose comparison technique can result in compromised passing rates when the treatment plan includes a high dose gradient perpendicular to the measurement plane. To address this, we have developed and tested a 3D gamma analysis tool.

Methods:An in-house 3D-gamma analysis algorithm was developed. Five VMAT patient specific QA plans were delivered and measured using a MatriXX 2D ion chamber array. A 3D dose volume was generated in the Pinnacle TPS, and exported to the analysis software. Both 2D and 3D gamma analysis were performed. The results determined using a 3%/3mm passing criteria were then evaluated and compared.

Results:Cases that did not include a high dose gradient perpendicular and adjacent to the detector plane showed only minor differences between the 2D and 3D gamma analysis results. Cases including a high dose gradient (e.g. as a means to avoid an adjacent sensitive structure), the use of 3D gamma analysis resulted in improved passing rates with lower gamma values. For the five cases tested, the average gamma index value was reduced from 0.361 to 0.345 when 3D analysis was used. The average passing rate increased from 94.7% to 96%.

Conclusion:A 3D gamma analysis tool has been developed and tested. For patient specific IMRT/VMAT QA performed using the composite dose measurement method, the use of 3D gamma analysis can effectively reduce the uncertainty in QA results due to factors such as setup errors. This technique can more effectively account for a high dose gradient perpendicular to the measurement plan. Consequently, improved and more meaningful QA results can be achieved using this technique.

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