A discrepancy in QA result may be caused by the CT-Ed data in planning system, investigation into the improvement of IMRT planning dosimetry verification

Full text: Ionization point dose measurements performed in routine intensity-modulated radiotherapy (lMRT) quality assurance (QA) have consistently indicated lower dose compared to treatment planning system (TPS) calculated results when performing the QA in a phantom other than water. An inconsistency in CT number to electron density (ED) conversion for non-water equivalent materials such as acrylic has been previously reported. While this problem may be avoided by performing QA in a water or equivalent material, consideration must be given to these results and their ability to represent the true planned dose as well as the TPS accuracy in handling tissue imhomogeneity. Method CT and ED data were re-calibrated against the results of dosimetry measurements in the acrylic IMRT phantom. Further adjustments were made based on physical measurements in differing materials covering the upper and lower limits of the CT -ED conversion. QA measurements of clinical step and shoot lMRT and VMAT cases was performed in both solid water and acrylic phantoms in order to test the adjusted CT-ED data. A 50% improvement in point dose QA results was achieved for most clinical IMRT and YMAT cases with the measured discrepancy being reduced from an average 3.0-4.0% to within 1.5-2.5% of TPS calculated result. The re-calibration of CT-ED data based on physical measurements can increase the accuracy of TPS equivalent depth conversion for inhomogeneous tissues and ultimately improve the agreement between TPS calculated and delivered dose during a step and shoot IMRT or YMAT treatment.