SU‐E‐T‐341: DVH‐Based Comparison of True 3D Measurements to a Delta4 System

Abstract

Purpose:Delta4 dosimetric software can be used to calculate DVH‐based metrics for patient‐specific quality assurance from measurements made by a Delta4 QA device. This study investigates the effectiveness of a novel transform method that transposes measurements made with a full‐density 3D dosimeter onto patient anatomy, enabling the calculation of DVHs. This allows for DVH comparisons from the transformed dose distribution, which are based on true 3D measurements, to those from the Delta4 system, which are based on semi‐3D measurements and interpolation.Methods:A double‐arc VMAT treatment for a head‐and‐neck case was delivered to a 1kg PRESAGE 3D dosimeter inserted into a polyurethane head phantom. The dosimeter was readout using an in‐house optical‐CT scanner to gather full‐density 3D dosimetric data. The transform method is achieved by multiplication of the measured doses with a “transformation matrix” which accounts for heterogeneities and differences in geometry between the patient and the phantom. The transformation matrix is a voxel‐by‐voxel division of the patient planned dose by the phantom planned dose, both calculated in the treatment planning system (Eclipse). The transformed distribution was then overlaid on the patient CT data, enabling the calculation of DVHs. The same VMAT treatment was delivered to the Delta4 phantom and DVH data was calculated using its associated software.Results:The transformed dose distribution showed good agreement with calculated patient values, determined by similarity in dose profiles between the two distributions and a 3D gamma index passing rate of 94.87% for 3%/3mm criteria. For every structure contained within the dosimeter volume, the transformed DVHs demonstrated better agreement than the Delta4 DVHs, when compared to the values calculated in the treatment planning system.Conclusion:The coupled technique of full‐density 3D dose measurements and the presented transform method enables clinical patient‐specific quality assurance data that is more accurate than the semi‐3D Delta4 system.This work was supported by NIH R01CA100835.