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The Effect of Jaw Hysteresis On Half-Beam Fields

G Johnson*, E Marshall , Northwest Medical Physics Center, Lynnwood, WA


(Wednesday, 8/1/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 9

Purpose: To develop a jaw calibration technique and independent jaw test tolerances, accounting for fluctuation of position for a Varian C-series linac.

Methods: The jaws were tested to determine how much their position fluctuates in three different scenarios. Motion direction was tested by opening or closing the jaw to the prescribed position at gantry 0° and again at gantry 270° with the jaw aligned in the direction of gravity. Positional dependence examined the jaw position at several points across its range of motion. Ten images were taken, using the EPID, of each jaw with a thick lead block opposite of the tested jaw. This was repeated at each tested position and the fifty percent intensity line was used to define the field edge. The resulting average and maximum deviations were used to set an abutment dose tolerance. An independent jaw test, with asymmetric fields matched at CAX, was evaluated before and after calibration to determine the abutment dose. The jaw’s position was calibrated by correlating the potentiometer readout with known distances from the dose difference and needed adjustment values were based off prior penumbra measurements.

Results: The jaws with the largest deviations for a 5cm distance moving at a combination of in and out to the location were X1 and Y2, with maximum deviations of 0.3mm and 0.5mm, respectively. Increasing the field size increased the uncertainty of the jaws by an average of 0.28mm. With the gantry at 270°, the jaws showed an average standard deviation of 0.06mm. The jaw calibration procedure changed the abutment dose variation from 17% to 8%.

Conclusion: The jaw hysteresis shows an average deviation of 0.12mm and a maximum of 0.50mm. The abutment dose varies at 2% per 0.1mm per jaw. A tolerance of ±10% is acceptable after calibration and shows better clinical results.


Not Applicable / None Entered.


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

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