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Inactivation of the Transverse Beam Steering Servo System and the Effect of Linac's Operating Dose Rates On X-Ray Beam's Center and Profile Characteristics

E Sham*, B Kassahun, Gibson Cancer Center (Duke Cancer Network), Lumberton, NC


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To verify the effect of changing dose rates on the 6-MV x-ray beam center's position and beam profile's uniformity for a linac operating under a disenabled servo control of the monitoring ionization chambers' signals for the transverse beam steering coils system.

Methods: The 6-MV radiation isocenter position was measured with a Winston-Lutz marker positioning device in conjunction with an EPID of the Elekta’s linac. Beam profiles were measured with a 2D ionization chambers array; the flatness and symmetry of the measured beam profiles were calculated using the array-associated QA software. All parameters measurements were repeated with three nominal dose rates of 100, 300, and 600 MU per minute (MU/min) at four linac's cardinal gantry angles.

Results: The Winston-Lutz QA results showed that the 6-MV radiation isocenter remained constant in position within the measurement uncertainty of ±0.1 mm on the radial plane, but was offset by a maximum shift of 0.95 mm on the transverse plane when the dose rates varied between 100 and 600 MU/min. Also, the beam uniformity study showed that the radial beam profiles were constant in shape and position, while the transverse beam profiles were tilted and experienced offset when the dose rate was decreased from 600 to 100 MU/min. For the (25×25)-cm² field, the overall change in the beam’s flatness and symmetry on the transverse plane was on the order of 1% and 2%, respectively, over this full range of operating dose rates.

Conclusion: When the linac operates under fully activated control servomechanism, constant x-ray beam characteristics are reproducibly established. When the control servo system is inactivated, we discovered a 1-mm-displacement in the beam isocenter’s position and also a maximum of 2% change in the flatness and symmetry for the 6-MV x-ray beam’s profile when the dose rate changed within the full operating range.


Linear Accelerator, Monitor Chambers, Dosimetry


TH- External beam- photons: Quality Assurance - Linear accelerator

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