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Investigating the Feasibility of Vertical CBCT Imaging Using the Varian TrueBeam LINAC


X Fave

X Fave1*, P Balter1, R Martin1, M Ahmad2, T Pan1, L Court1, (1) UT MD Anderson Cancer Center, Houston, TX, (2) Stanford University, Stanford, CA

SU-E-J-168 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
To assess the TrueBeam LINACs imaging capabilities for taking vertical CBCT in order to provide an alternative treatment position for radiation therapy patients.

Methods:
The TrueBeam LINAC was programmed to take upright (vertical) CBCT images as follows: The LINAC gantry was positioned at 0 degrees with the x-ray tube and detector in lateral positions. The x-ray tube remained stationary and took fluoroscopic (projection) images as the couch rotated 180 degrees from 90 to 270 degrees. The x-ray tube current was normalized to give the same dose (measured using a calibrated Farmer chamber), as that received during a clinical scan, to the center of a cylindrical, polyethylene phantom. Images were taken of CATPhan and anthropomorphic phantoms and reconstructed using in-house CBCT reconstruction software. For thoracic scans, the detector was moved 10cm in the superior direction before rotating the couch 180 degrees. Then the detector was moved 10cm in the inferior direction and the couch rotated 180 degrees again. This was done to increase our field of view. For each reconstruction, spatial linearity and image quality were compared.

Results:
CBCT images were reconstructed and compared for horizontal and vertical scans. Spatial linearity was measured to be correct within 1mm for vertical CBCT images. Image quality decreased in reconstructed vertical CBCT images from that achieved in clinical mode for supine patients. Reasons for this included reduced scan angle and lack of a bow-tie filter in upright CBCT imaging.

Conclusion:
We studied the feasibility of using the TrueBeam LINACs imaging capabilities to take CBCT images of patients in an upright position. Research is ongoing to improve image quality in order to establish a clinical protocol for treatment planning and patient setup. Future plans include adding a bow-tie filter and adjusting for drift in the couch rotation through the use of a pin phantom.

Funding Support, Disclosures, and Conflict of Interest: This project was partially funded by Varian Medical Systems.

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