AbstractID: 9477 Title: Patient Setup with Respiratory Gated Electronic Portal Imaging
The effectiveness of using an Electronic Portal Imaging Device (EPID) for the setup of patients with a moving tumors depends on the
breathing phase at which the portal images are obtained. The potential setup error due to tumor motion can be as large as the peak-topeak amplitude. The 3-D coordinates of gold seeds, implanted near the tumor, can be found from a breath-hold or retro-gating CT scan
done for all breathing phases prior to treatment. During patient setup, EPID images can be obtained at exhale using the Real-time
Positioning Management (RPM) system from Varian Medical Systems. The 3D coordinates of the seeds, relative to machine isocenter,
can be found from a pair of images, using Varian’s Markervision software. Since the digital images will be obtained at the same point
in the breathing cycle as the reference CT scan, an appropriate, precise correction of patient position can be implemented. Phantom
studies have been performed using three gold seeds (0.8 mm x 3.0 mm) and 25 cm of solid water placed on a motor driven moving
platform. The motion is sinusoidal in the cranial-caudal direction, with a peak-to peak amplitude of 2 cm. Five EPID images were
taken at exhale and compared to a reference static exhale image. We found a mean deviation from the reference of 0.49 mm with a
variance of 0.24 mm (systematic errors ignored). The phantom study shows the viability of this technique for the positioning of
patients with moving tumors.
Research supported by Varian Medical Systems, Inc.