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Machine Specific Quality Assurance Procedure for Stereotactic Treatments with Dynamic Couch Rotations

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B Wilson

B Wilson1*, E Gete2 , (1)University of British Columbia ,Vancouver, BC (2) Vancouver Cancer Center, Vancouver, BC


SU-I-GPD-T-548 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: We present a method in which the treatment couch's accuracy is measured using the electronic portal imaging device (EPID) and a phantom of our own construction. Using this phantom, we were able to quantify the treatment couch walkout, and rotation angle accuracy for both static and dynamic couch treatments. These measurements were used to provide an accurate measure of the treatment couch walkout as well as verify the trajectory log.

Methods: The phantom was constructed using a polystyrene slab in which five stainless steel BBs of 4mm diameter are placed on the same plane at varying radii (0, 2.8, 4.4, 5.6, 6.7cm). The couch was rotated at max velocity through its full extent (-90, 90degrees) while MV images were acquired continuously. The couch rotation walkout was calculated using a least squares minimization which fit the locations of the BBs to their locations at reference setup conditions. Using this approach, rotation angle and isocentre walkout was calculated in 3 dimensions. These measurements were used to quantify the accuracy of the couch as well as to validate the Varian TrueBeam trajectory logs.

Results: The measured couch center of rotation consisted of a cloud of points clustered around the room isocentre within ≤0.7mm distance. The trajectory log couch angle values agreed with those recorded in the dicom header files (EPID images) to the third digit of accuracy. Additionally, the couch values recorded in the trajectory log and dicom header agreed with the calculated values to 0.08 degrees.

Conclusion: We have developed a quality assurance method for the treatment couch for dynamic and static deliveries which is simple, accurate and enables the user to access a multitude of consistent data with a single measurement. Using this method, we have shown that the treatment couch is accurate for both static and dynamic stereotactic deliveries.

Funding Support, Disclosures, and Conflict of Interest: The authors would like to disclose that the work was partially supported by the BC Cancer Agency as well as Varian Medical Systems

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