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Calibration of a 2D Planar Diode Array with An Integrated Ion Chamber

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J Barbiere

J Barbiere*, J Napoli , A Ndlovu , Hackensack Univ Medical Center, Hackensack, NJ


SU-I-GPD-T-297 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Reports of high rate of failure in credentialing and lack of sensitivity to known errors may be related to known effective depth (Deff) calibration discrepancies for patient specific quality assurance (QA) with planar diode arrays. This work presents a method incorporating a micro ion chamber (MIC) embedded in solid water above the array for calibration independent of Deff.

Methods: Initial array reading-to-dose calibration (CalDepth) is performed based on a calculated dose at the geometric setup depth. A Superposition-Convolution treatment planning algorithm is used to compute patient plan array dose (PAD) distributions in the detector plane for patient fields and the MIC dose (PCD). After a measurement, a normalization actor NF = CMD/PCD is calculated based on the chamber measured dose (CMD).The array measurement dose (AMD) on the central ray is also related to PAD at the corresponding area by NF = AMD/PAD. Therefore the array measured dose is rescaled to AMD = PAD*NF by editing the calibration factor in the measured dose text file to CalChamber = CalDepth/NF. Gamma analysis QA evaluation was performed with parameters 3% and 1% for percent dose difference (%DD) and 3mm and 1mm for distance to agreement (DTA). A static field was measured at 100% and 98.5% delivered dose to test sensitivity to known error. An IMRT field was also measured with Deff and MIC calibration. Gamma passing rate and dose difference histograms were generated.

Results: The method sensitivity is able to detect 1.5% changes in delivered dose. Histograms are centered accurately within 0.5% on measured dose and have reduced normal distribution standard deviation. The 2%/1mm gamma passing rate for one IMRT field increased from 66.9% to 95.0%.

Conclusion: The addition of an integrated ion chamber calibration with diode arrays makes it possible to significantly tighten gamma parameters thereby increasing sensitivity and accuracy of patient specific QA.

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