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Reliability Study of ExacTrac- System Image Isocenter Using An On-Demand QA Procedure


X Wang

X Wang1*, Z Zhao1, W Du1, X Li1, J Yang1, (1) UT MD Anderson Cancer Center, HOUSTON, TX

SU-E-J-49 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
To develop a pointer based quality assurance (QA) procedure that quickly verifies the congruence of ExacTrac® image isocenter with the radiation isocenter of a linear accelerator (linac), and use it to study the accuracy and reliability of the ExacTrac® image isocenter.
Method and Materials:
A 6 mm diameter tungsten ball was aligned with the room laser using a pointer alignment system. By measuring the positions of ExacTrac® image isocenter and linac radiation isocenter relative to the ball, the deviations between the two isocenters was easily calculated. The ExacTrac® image isocenter relative to the ball was measured using the built-in Winston-Lutz Pointer test, in which kV images of the ball are projected to flat panels and the center of the ball to the image isocenter are calculated by ExacTrac® software. The linac radiation isocenter relative to the ball was measured by taking the portal images of the ball at four gantry angles, 0, 90, 180 and 270 with a 6 x 6 cm² field. An in-house developed software was used to identify the center of the ball and the center of the radiation. This procedure was used in weekly QA for a year in this work.
Results:
A simple QA procedure was developed to ensure the accuracy of ExacTrac® image isocenter against the linac radiation isocenter. The procedure can be executed in less than 20 minutes. The data show the position error were -0.27±0.30, 0.52±0.24, 0.18±0.22 mm in the lateral, vertical and longitudinal directions, and the max deviation were 0.86, 0.89 and 0.54 mm respectively.
Conclusion:
The weekly verification data showed the ExacTrac® image isocenter can be very reliable with sub-millimeter accuracy. This level of imaging alignment accuracy is necessary for imaging guided SBRT procedures. The quick QA procedure can be applied on-demand to ensure the accuracy of ExacTrac® isocenter.

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