MO‐E‐17A‐11: In‐Phantom Detector Response in a Kilovoltage X‐Ray Beam

Abstract

Purpose:To investigate changes in detector response in a kilovoltage x‐ray beam as a function of depth in phantom.Methods:Detector response in a 120kVp x‐ray beam as a function of depth in a PMMA phantom was determined for a reference class Exradin A12 ionization chamber, an Exradin A1SL ionization chamber, TLD‐100 and EBT3 radiochromic film and normalized to a depth of 1cm. The output of each detector was referenced to the A12 ionization chamber reading to assess changes in detector response as a function of depth. The BEAMnrc, DOSxyz, and egs_chamber user codes of the EGSnrc Monte Carlo code were used to calculate the dose delivered to the detectors from an x‐ray source with similar beam quality to the one used in the measurements.Results:Changes in detector response relative to the A12 ionization chamber of 2%, 7%, and 14% for the A1SL, TLDs, and film, respectively, were observed with the larger changes in response occurring at larger depths. The Monte Carlo calculated changes in dose response showed less deviation, with the largest difference being within 2%.Conclusion:The Monte Carlo simulations show that doses delivered to the different detectors are quite similar, while measurements show that the detector output varies drastically. This indicates that the detector output per unit dose is changing with depth, most likely due to changes in the photon spectrum that occur because of scatter and attenuation of the primary beam. Because detector calibration is often performed at a single point in the kilovoltage energy range, the change in detector response needs to be considered when the measurement conditions differ from those of the calibration. This is of particular concern in computed tomography dose measurement, where calibrations are performed at shallow depths or in air, and measurements are made at deeper locations.