Reduced Computed Tomography Scan Speed Improves Alignment Errors for Patients Undergoing Thoracic Stereotactic Body...

Area of Science:

• Radiation Oncology
• Medical Imaging
• Image-Guided Therapy

Background:

• Motion artifacts in computed tomography (CT) imaging can lead to alignment errors during image guidance for thoracic stereotactic body radiation therapy (SBRT).
• CT-on-rail (CTOR) is used for image guidance, but its accuracy can be compromised by patient motion, particularly respiratory motion.

Purpose of the Study:

• To evaluate the effectiveness of a slow CT protocol in minimizing alignment errors caused by motion during CT-on-rail (CTOR) image guidance for thoracic SBRT.
• To compare the performance of standard and slow CT protocols in reducing longitudinal coordinate discrepancies.

Main Methods:

• A lung phantom with simulated tumor motion (varying breathing rates and amplitudes) was scanned using both standard and slow CT protocols.
• Image guidance was performed using in-house software, comparing alignment accuracy between the two CT protocols.
• Retrospective analysis of cone-beam CT (CBCT) data from 31 SBRT patients was conducted to assess longitudinal alignment systematic errors.

Main Results:

• The slow CT protocol demonstrated improved alignment accuracy, particularly at higher motion amplitudes and slower breathing rates (e.g., 1.6 mm vs. 5.2 mm at 8 BPM/15 mm amplitude).
• Retrospective patient data revealed systematic differences in alignment between simulation and daily CBCT, indicating vertebral bodies are unreliable surrogates for tumor position.
• A planning target volume (PTV) of 5 mm was insufficient for tumors with motion exceeding 9 mm.

Conclusions:

• A slow CT protocol enhances alignment precision for thoracic SBRT, though slower breathing rates present greater challenges.
• Accurate longitudinal tumor positioning requires determination from multiple CTOR images, not solely relying on bone structures.
• Averaging coordinates from multiple CTOR images is recommended for optimal patient alignment in thoracic SBRT.