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Evaluation of the Geometric Accuracy and Image Quality of KV CBCT Based IGRT System

P Ravindran

P Ravindran*, S Kumar, T Peace, Christian Medical College, Vellore, Tamil Nadu

SU-E-J-162 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the geometric accuracy and image quality of kV CBCT based image guided Radiotherapy system.

Methods: A cube phantom with a radio opaque marker was used to determine the geometric accuracy of the isocenter of the CBCT unit. The cube phantom is positioned such that the radio-opaque ball is at the isocenter. Radiographic images were acquired at various gantry angles and the ball center is matched with the imager center on the OBI software. The image quality assessment was performed with a CatPhan phantom that has objects to asses the low contrast resolution, uniformity of HU, geometric accuracy and special resolution. CBCT images of the phantom were acquired for Full Fan beam (FF) and Half Fan beam (HF) for large field of view. The radiation dose was estimated with a CTDI pelvic phantom

Results: The maximum isocentre shift estimated with the Winston-Lutz test tool was <1mm for most gantry angles. The maximum variation in the HU uniformity was 15 in one region and the rest were within 10 units for FF beam. The special resolution was 8lp /mm for FF beam and 4lp/mm for HF beam. The low contrast resolution, about 5 of the 1% supra slice discs were seen and about two .5% supra slice discs were seen with FF beam. CBCT images of the CatPhan with FF beam during a regular QA showed a ring artifact that was not reflected in patient images. This was identified to be due to a shift in the position of the bow-tie filter that was subsequently corrected.

Conclusion:This work shows that the CBCT image guidance system is suitable for position verification in Radiotherapy application. However, it is essential to have a periodic quality assurance program for CBCT in place to identify issues due to mechanical misalignment

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