Evaluation of the need for simultaneous orthogonal gated setup imaging

Area of Science:

• Medical Physics
• Radiotherapy
• Image Guidance

Background:

• Image-guided patient setup for radiotherapy commonly uses sequential, respiratory-gated orthogonal radiographs.
• This asynchronous imaging approach may introduce errors due to changes in internal/external correlation during the acquisition interval.

Purpose of the Study:

• To quantify the setup error introduced by asynchronous (non-simultaneous) acquisition of orthogonal radiographs in respiratory-gated radiotherapy.
• To evaluate the impact of varying time intervals between sequential radiographic acquisitions on the accuracy of patient positioning.

Main Methods:

• A computational simulation was performed using data from eight patients undergoing SBRT, including internal and external coordinates acquired at 30Hz.
• The simulation modeled the position of an implanted fiducial marker in asynchronous orthogonal image sets with time intervals of 30, 60, and 90 seconds.
• Setup position was calculated using a combination of initial (AP) and final (LR) simulated 2D images, and the 3D error was compared to the reference position.

Main Results:

• The mean 3D error was consistently low across time intervals: 0.8 mm for 30s, 0.8 mm for 60s, and 0.6 mm for 90s.
• The 3D error was less than 1 mm for 77% of data points, less than 2 mm for 89%, and less than 3 mm for 98%.
• No significant correlation was found between the time interval between images and the resulting 3D setup error.

Conclusions:

• Asynchronous acquisition of orthogonal radiographs for respiratory-gated patient setup results in small, clinically acceptable setup errors.
• The timing interval between sequential images, up to 90 seconds, does not significantly impact the accuracy of patient positioning.
• These findings support the use of non-simultaneous imaging for patient setup in radiotherapy, provided careful consideration of potential internal/external motion correlation.