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An Investigation of Patient Plan-Specific Correction Factors for Three Ion Chambers in Collapsed Composite IMRT Treatments Using a PTW MicroDiamond and Monte Carlo Simulations

V Desai*, Z Labby , W Culberson , university of Wisc Madison, Madison, WI


(Thursday, 8/2/2018) 1:00 PM - 3:00 PM

Room: Karl Dean Ballroom A1

Purpose: To investigate the suitability of the PTW microDiamond detector as a means of correcting chamber response in modulated IMRT fields for three different ionization chambers through both measurements and Monte Carlo simulations.

Methods: Three clinical IMRT plans were imported and converted to EGSnrc Monte Carlo inputs using validated models of the Varian Clinac 21EX and three Exradin ionization chambers. Full treatment simulations incorporating all collimator and MLC motions but with the gantry held stationary at 0â?° were delivered to the three ionization chambers as well as a PTW microDIamond in a water tank geometry. The chamber response correction factors (kQ,clin) were determined through both measured and simulated signal/dose ratios and compared.

Results: Monte Carlo models were assessed by comparing response ratios of both simulated and measured response in each step-and-shoot IMRT field to a set of reference field conditions. The root mean squared error between measured and simulated response ratios for the Exradin A12 Farmer-type chamber, Exradin A1SL scanning chamber, Exradin A26 microchamber, and PTW microDiamond were found to be 0.4%, 0.7%, 1.1%, and 0.9%, respectively. Measured and simulated kQ,clin factor agreement was mixed across all detectors investigated.

Conclusion: Small changes in the measured or simulated detector response ratio can greatly affect the magnitude of the derived correction factor. The absorbed dose to water remains the most difficult quantity to simulate or measure and the presence of steep gradients in highly modulated IMRT plans further complicates matters. Despite its suitability for use in small fields, the PTW microDiamond (and any other potential small field detector) must be carefully characterized for use in nonstandard composite fields.


Absolute Dosimetry, Linear Accelerator, Small Fields


TH- External beam- photons: Small/nonstandard field experimental dosimetry

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