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Evaluation of MV and KV Isocenter Displacement for An IGRT Linac with and Without An Add-On Micro-Multileaf Collimator

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R Nakada

R Nakada1,2*, D Tatsumi1, A Takemura3, A Ienaga1, A Yomoda1, M Inoue1, T Ichida1, M Hosono4, (1) Osaka City University Hospital, Osaka, Japan, (2) Division of Health Sciences, Graduate School of Medical Science , Kanazawa University, Ishikawa, Japan, (3) Faculty of Health Sciences , Institute of Medical , Pharmaceutical and Health Sciences, Ishikawa, Japan, (4) Department of Radiology , Osaka City University Graduate School of Medicine, Osaka, Japan

SU-E-J-14 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The impact of the add-on micro MLC (mMLC) on the MV and kV isocenters was investigated.

Methods:Winston-Lutz tests was performed with an add-on mMLC, and a ball bearing (BB) was placed at the isocenter given by localization lasers. A square MV field was shaped by the mMLC, and the MV field center position relative to the BB position were measured by a portal imager with different gantry angles. A desirable BB position that minimizes the maximum deviation was obtained as an MV isocenter, and the BB center was aligned onto the revised MV isocenter. Using an on-board kV imaging system, two-dimensional BB image was acquired during gantry rotation. The displacement of the kV projection image center from the BB for each gantry angle was recorded as 'flexmap'. After unmounting the mMLC, Winston-Lutz test and the flexmap measurement were performed again. A phantom was placed on a couch and cone-beam CT (CBCT) images were acquired with the two different flexmap data. The set-up errors were calculated and the differences of required couch translations with each of the flexmap data were obtained.

Results:After unmounting the mMLC, the MV field center and the kV image center were shifted approximately 0.4 mm and 0.05 mm upward , respectively. Assuming a perfect MV and kV isocenter coincidence with mMLC using the flexmap for CBCT reconstruction, the MV/kV isocenter discrepancy without the mMLC could be calculated by the difference of the above two values, 0.35 mm which satisfies AAPM TG179 guideline of 1 mm. The largest difference of required couch translations between the two flexmap data was 0.07 mm toward the vertical direction, which agreed well with the kV image center shift of 0.05 mm.

Conclusion:The MV and kV isocenters defined with the add-on mMLC are also applicable to image-guided VMAT without the mMLC.

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