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A Rapid Joint Therapist-Physicist QA Procedure for Sub-Mm Verification of KV Imaging System-Radiation Isocenter Alignment for Frameless SRS

E Huang

E Huang1*, P Wang1, J Mechalakos1, G Li1, A Ballangrud Popovic1, l kuo1, D Lovelock1, (1) Memorial Sloan Kettering Cancer Center, New York, NY

SU-E-J-18 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

To assess the feasibility of a rapid and accurate joint therapist-physicist QA procedure to verify the accuracy of the alignment kV imaging system with the MV radiation Isocenter prior to frameless SRS treatment.

A Winston-Lutz phantom was CT scanned with a slice spacing of .75 mm. The isocenter was placed to within 0.1 mm of the center of the sphere. A plan was created containing 4 static 2 x 2 cm MLC fields with corresponding MV portal images, and 4 kv setup fields both spaced at 90d gantry angles and CBCT setup field. Prior to the frameless SRS treatment, therapists place the WL phantom approximately at isocenter, then acquire all images planned.
A physicist extracts the images from the R&V system and performs an automated image analysis to compute the systematic error between the radiation isocenter and the origin of the radiographic and CBCT systems. Spatial accuracy was assessed by placing a steel sphere on a micrometer stage and making known dispalcements.
The therapists will do 3D-3D match between acquired CBCT and planned CBCT and record the couch shifts. The planar MV and KV images are analyzed automatically off-line to determine the systematic error between the kV imaging origin and the radiation isocenter before SRS treatment.


The localization uncertainty of the sphere, for both the KV and MV systems was ± 0.1mm. Uncertainty on the systematic error between the radiation isocenter and the imaging origin was found to be ±0.2 mm. The time required to verify the spatial imaging accuracy required about 15 minutes of therapist time and about 10 minutes of physicist time.
The designed QA procedure can be performed easily by the therapists and physicists to speed up SRS room QA before patient treatments.

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