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Session: Therapy General ePoster Viewing [Return to Session]

Evolution and Cause of Rotation-Induced Couch Shift for a New SRS-Capable Linac

X Zhai*, Y Liang, M Zhang, L Voros, H Zhang, B Mueller, Y Song, Memorial Sloan-Kettering Cancer Center, New York, NY


PO-GePV-T-373 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Rotation-Induced Couch Shift (RICS) is one of the essential mechanical metrics of a Linac-based SRS delivery system. A tolerance of ≤1.0mm is required for accurate SRS treatment delivery. For TrueBeam SRS Linacs, Varian recommends a tolerance ≤0.75mm. This parameter could exceed the tolerance as the couch ages. For the past two years, we have followed the evolution of this parameter for a new TrueBeam SRS Linac while pending state inspection and approval and attempted to diagnose the cause of RICS.

Methods: RICS is a vector quantity. Its direction points from the isocenter to maximum shift in 3D space. Its magnitude is the square root of LAT2+LNG2+VRT2, where LAT, LNG, and VRT represent couch shift in each dimension, respectively. To measure RICS, we performed monthly Beam and Geometry Checks with Enhanced Couch feature using MPC. Enhanced Couch MPC acquired MV images at couch angles of 270⁰, 315⁰, 354⁰, 360⁰, 6⁰, 45⁰, and 90⁰ and kV images at couch angles of 354⁰ and 6⁰. Measured RICS was then analyzed.

Results: Although our TrueBeam SRS Linac has not been in clinical service, it was found that RICS had been continuously increasing, from the baseline of 0.21mm at the time of commissioning to the most recent reading of 0.66mm. Linear regression showed a strong positive linear relationship between RICS and time, with R2=0.8239. According to this model, RICS will exceed the vendor-recommended tolerance in a few months. Since our SRS Linac has not been in use, we hypothesize that RICS could have been caused by machine stand movement.

Conclusion: For TrueBeam SRS Linacs not in clinical service, regular machine performance checks with Enhanced Couch MPC are imperative to maintain optimal operating conditions. Data trends and patterns revealed by Enhanced Couch MPC could serve as prognostic indicators for preventative machine maintenance.

Funding Support, Disclosures, and Conflict of Interest: This project is funded by the Rose-Margulies Family Research Fund.


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