Target localization accuracy in a respiratory phantom using BrainLAB ExacTrac and 4DCT imaging.

Area of Science:

• Medical Physics
• Radiotherapy Technology

Background:

• Accurate internal target motion measurement is crucial for effective gated radiotherapy.
• Four-dimensional computed tomography (4DCT) and ExacTrac X-ray imaging are used for this purpose, but their accuracy varies.

Purpose of the Study:

• To evaluate and compare the accuracy of 4DCT and ExacTrac X-ray imaging systems in measuring internal target motion using a respiratory phantom.

Main Methods:

• A respiratory phantom with a metal coil surrogate target underwent sinusoidal motion (5-25 mm amplitude, 4.0s period).
• 4DCT and ExacTrac images were acquired at specific respiratory phases.
• Coil displacements and length distortions were measured and compared to known values.

Main Results:

• ExacTrac showed a maximum localization error of 0.8 mm, while 4DCT had a maximum error of 3.5 mm.
• 4DCT exhibited significant coil length distortion (up to 8.3 mm) at mid-respiratory phases with high motion velocity.
• ExacTrac accurately localized the coil across all respiratory phases without observable distortion.

Conclusions:

• The ExacTrac X-ray imaging system provides superior accuracy for internal target motion measurement compared to 4DCT.
• ExacTrac minimizes uncertainties in target size and motion, potentially reducing internal target volume margins in gated radiotherapy.