Validation of the path integration of 4D dose distributions with 4D phantom measurements

Abstract

4D treatment planning generates a series of 3D dose distributions to represent the motion properties of the patient and/or radiation delivery system. The purpose of this study is to validate with 4D phantom measurements the use of the 3D optical flow method algorithm for the numerical path integration of 4D dose distribution data into a single 3D dose distribution. The Radiological Physics Center (RPC) thoracic phantom was placed on a programmable motion table to simulate respiratory motion. 4D CT imaging was performed on a multi-slice CT scanner (Philips Medical Systems, Anhover, MA). The CT data was binned into eight phases. The CT images were sent to a Pinnacle workstation (version 6.2b, Phillips Medical Systems, Anhover, MA) where a treatment plan was designed on the end expiration image set. The treatment plan was transposed onto the other respiratory phases and a 4D dose data set generated. The plan was delivered using a Varian 2100 linear accelerator (Varian Medical System, Palo Alto, CA). The dose delivered to the RPC thoracic phantom was recorded using radiographic film and thermoluminescent dosimeters for calibration. The 3D optical flow method (3D OFM) algorithm is a deformable image registration algorithm we have implemented. 3D OFM was used to calculate the displacement fields between the respiratory phases. The displacement fields were used to map the dose distributions from the multiple respiratory phases onto the end expiration phase. The resulting dose distributions were summed with equal weighting, performing numerical path integration. The calculated and measured dose distributions were compared with the static plan. The attached figure illustrates the dose distributions. A coronal section through the static treatment plan is shown in figure A.u1/2pick;3197f1;0;11 The 3D OFM algorithm was applied and numerical path integration performed on the 4D dose distribution data resulting in a single 3D dose distribution. The corresponding coronal section is shown in figure B. A coronal section through the measured dose distribution is shown in figure C. The delivered dose distribution had a peak width at 90% maximum of 38.5 mm. The corresponding value for the path integrated 4D plan and the end expiration static plan were 42.6 mm and 47.0 mm. The 50% isodose line was displaced 0.5 mm when compared with the path integrated 4D plan and 8.6 mm when compared with the end expiration static plan. The 3D OFM algorithm provides the displacement field necessary for numerical path integration to map a 4D dose distribution data set into a single 3D dose distribution accurately. The resulting dose distribution was found to more closely represent the delivered dose versus the static plan for a moving phantom.