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Long-Term Performance of a Multi Layer Ion Chamber (MLIC) for Proton QA


V Devaraju

V Devaraju*, R Slopsema, University of Florida Proton Therapy Institute, JACKSONVILLE, FL

SU-E-T-80 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
To evaluate the performance of a Multi Layer Ion Chamber (MLIC) in proton-therapy quality assurance measurements
Methods:
To monitor the output of proton beam delivery system, measurement protocols were developed for two reference fields having a range of 15.1 g/cm² and 25 g/cm² in water. Measurements were performed at periodical interval using a proto type multi-layer ion chamber (MLIC) developed by Ion Beam Application (IBA), Belgium, as well as with parallel plate ion chamber placed in water phantom, on three proton gantries over a period of one year. The resulting parameters such as range, modulation and output factor were compared. Besides, measurements for patient specific treatment fields having wide spectrum of range and modulation width were conducted and the percentage difference between the modeled output and measured output was compared with the results obtained using MLIC and water phantom.
Results:
From the large number of data set collected, the comparison results demonstrated that almost all measurements both with MLIC and water phantom, with exception of few, were within the prescribed limits (Range & Modulation < ±3mm and output < ±3%). However, a general trend of slightly lower value for range and output factor and higher value for modulation width in MLIC measurements in comparison to water tank measurements was observed. Uncertainty in MLIC calibration and the effect the MLIC aperture are likely causes for this deviation.
Conclusions:
Multi-layer ion chamber was generally stable for QA measurements and very convenient to use to obtain quick measurements within an acceptable uncertainty, compared to water phantom method of measurements. However, for a higher accuracy and base line commissioning data, ion chamber measurements in water phantom will continue to be the preferred choice of measurement methodology.

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