TU-E-BRA-11: Volume of Interest Cone Beam CT with a Low-Z Linear Accelerator Target: Proof-of-Concept

Area of Science:

• Medical Physics
• Radiological Imaging
• Radiation Oncology

Background:

• Cone beam CT (CBCT) is widely used in radiation therapy for image guidance.
• Traditional CBCT employs full-field imaging, potentially leading to unnecessary dose exposure to healthy tissues.
• Developing targeted imaging techniques is crucial for dose reduction in radiotherapy.

Purpose of the Study:

• To demonstrate the feasibility and advantages of volume of interest (VOI) CBCT.
• To evaluate the use of a 2.35 MeV electron beam on a carbon target for VOI CBCT.
• To assess dose reduction and image quality metrics of this novel approach.

Main Methods:

• Utilized a Varian 21EX linear accelerator with a 2.35 MeV electron beam and a carbon x-ray target.
• Defined arbitrary imaging volumes (VOIs) for dynamic MLC tracking.
• Employed a priori DRR information to mitigate truncation artifacts.
• Performed phantom and ex vivo tissue imaging.
• Measured dose distributions and Contrast-to-Noise Ratio (CNR) using radiochromic film and phantom measurements.

Main Results:

• The 2.35 MV/Carbon beam demonstrated favorable CNR characteristics.
• DRR filling effectively eliminated truncation artifacts.
• Imaging dose was reduced by 5-10% inside and 75% outside the VOI compared to full-field imaging.
• CNR remained invariant with VOI size, indicating dose reduction as the primary benefit.

Conclusions:

• VOI CBCT using a 2.35 MV/Carbon beam is a feasible technique for targeted imaging.
• This method allows for dose localization to the selected VOI.
• The primary advantage lies in substantial imaging dose reduction, not necessarily improved image quality.