3D fluoroscopic image estimation using patient-specific 4DCBCT-based motion models.

3D fluoroscopic images represent volumetric patient anatomy during treatment with high spatial and temporal resolution. 3D fluoroscopic images estimated using motion models built using 4DCT images, taken days or weeks prior to treatment, do not reliably represent patient anatomy during treatment. In this study we developed and performed initial evaluation of techniques to develop patient-specific motion models from 4D cone-beam CT (4DCBCT) images, taken immediately before treatment, and used these models to estimate 3D fluoroscopic images based on 2D kV projections captured during treatment. We evaluate the accuracy of 3D fluoroscopic images by comparison to ground truth digital and physical phantom images. The performance of 4DCBCT-based and 4DCT-based motion models are compared in simulated clinical situations representing tumor baseline shift or initial patient positioning errors. The results of this study demonstrate the ability for 4DCBCT imaging to generate motion models that can account for changes that cannot be accounted for with 4DCT-based motion models. When simulating tumor baseline shift and patient positioning errors of up to 5 mm, the average tumor localization error and the 95th percentile error in six datasets were 1.20 and 2.2 mm, respectively, for 4DCBCT-based motion models. 4DCT-based motion models applied to the same six datasets resulted in average tumor localization error and the 95th percentile error of 4.18 and 5.4 mm, respectively. Analysis of voxel-wise intensity differences was also conducted for all experiments. In summary, this study demonstrates the feasibility of 4DCBCT-based 3D fluoroscopic image generation in digital and physical phantoms and shows the potential advantage of 4DCBCT-based 3D fluoroscopic image estimation when there are changes in anatomy between the time of 4DCT imaging and the time of treatment delivery.

SU-E-J-148: Fabrication of an Anatomically Realistic Dynamic Respiratory Phantom.

PURPOSE: The purpose of this project was to design an anatomically correct respiratory phantom that allows for accurate dose measurement within thoracic structures that move in a realistic fashion allowing for a more accurate simulation of in vivo measurements. METHODS: The basis for this phantom is The Dynamic Breathing Phantom (TBP), an Alderson Phantom by Radiology Support Devices (RSD). The phantom was disassembled, removing the proprietary pneumatic lung apparatus as well as the motor driven tumor mount. A CT of the phantom at rest in the exhale position was acquired with 0.25 cm slice thickness. Every sixteenth slice was printed out to scale. The lung cavity was segmented on the images. These were used to cut out 4.0 cm thick slabs of foam rubber matching the contour of the lung. These were assembled along with other thoracic structures manually imbedded in the foam rubber. The organs were then placed within the chest cavity of the phantom. A purpose-built diaphragm chamber made of a rubber bladder was inserted under the lung material. The manufacturer-provided air compressor system was re-tasked to drive the diaphragm chamber. RESULTS: The foam rubber used as lung material has comparable density to human lung (-800 HU). The phantom is capable of producing realistic respiratory motion. This phantom will easily accommodate a variety of dosimeters and can be adapted for a variety of tumor/critical structure shapes, sizes and locations. CONCLUSIONS: The creation of this versatile humanoid phantom opens the door for a multitude of experiments to investigate dose to organs within the chest cavity for different planning techniques, under different respiratory condition, while using a more anatomically correct experimental setup.