Program Information
OBI Blade Calibration Verification On TrueBeam 2.5 (What To Do If It Fails)
S Klash1*, (1) Wesley Medical Center, Wichita, KS
Presentations
SU-I-GPD-J-41 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To verify if the OBI kV collimator's blade position calibration is within Varian's specification of ±2.5mm per blade and, if a blade falls outside specification, how to adjust the blade, if possible.
Methods: Linac gantry was positioned at o degrees (AP) and the blade collimation plate was setup on the treatment couch with the printed side of the board facing the treatment head. The plate was centered to the linac crosshair at an SSD of 100cm. The gantry was then rotated so the kV source was directly above the blade calibration plate. Two kV collimator field sizes were used for testing and various kV, mAs technique factors were used to image the plate. The digital graticule on the OBI workstation was used to align the plate. The OBI workstations measurement tool was used to measure the distance from the edge of the x-ray field (50% penumbra) to the expected collimator position defined by the markings on the calibration plate.
Results: Two blades failed to meet the Varian specification no matter what exposure technique was used. A kV collimator initialization was performed without any change in result. Next, a kV Collimator calibration was performed which indicated three of the four OBI collimator blades exceeded the ±2.5mm tolerance. The OBI workstation's viewing window and levels were darkened per Varian recommendation which resulted in the swapping of the blades that were out of tolerance with the blades that were in tolerance.
Conclusion: When the OBI kV collimator's blade position is outside Varian's published tolerance, Varian service personnel do not have a definitive way of correcting this issue outside of completely taking the kV collimator assembly apart. Adjusting window and levels, per Varian recommendation, to make the blade accuracy test result "look acceptable" appears to not be a solution.
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