SU-E-J-33: Geometric Agreement Check for Imaging System with Radiation Beam by KV and MV-CBCT

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-Guided Therapy

Background:

• Accurate geometric alignment is crucial for effective radiation therapy.
• Current quality assurance (QA) methods for image-guided radiation therapy (IGRT) systems can be time-consuming.
• Verifying the geometric agreement between IGRT systems and treatment beams is essential for patient safety.

Purpose of the Study:

• To develop a novel, rapid QA method for assessing geometric agreement in kV-IGRT systems.
• To confirm the temporal stability of geometric alignment between the kV-IGRT system and the treatment beam.
• To utilize both kV- and MV-CBCT for enhanced geometric verification.

Main Methods:

• Sequential MV-projection images were acquired during gantry rotation and reconstructed into MV-CBCT with flexmap correction to account for gantry sag.
• The accuracy of MV-CBCT reconstruction was evaluated using a 8mm ball-bearing (BB) at the isocenter with simulated gantry/detector displacements.
• kV-CBCTs from the XVI system were compared with MV-CBCTs of the BB to confirm geometric agreement.

Main Results:

• MV-CBCT reconstruction successfully identified geometric shifts of the MV-detector panel and gantry, with detectable changes as small as 1mm.
• Comparison of BB images between kV-CBCT and MV-CBCT showed a discrepancy of less than 1mm in geometric center.
• The proposed method demonstrated the ability to recognize geometrical changes in the MV-detector panel.

Conclusions:

• The novel QA method using kV- and MV-CBCT is effective for detecting geometric non-idealities in kV-IGRT systems.
• This approach provides a simple and quick verification of geometric agreement between the IGRT system and the treatment beam.
• The method can identify geometrical changes less than 1mm, ensuring treatment accuracy.