Development and evaluation of an ultrasound-guided tracking and gating system for hepatic radiotherapy

Respiratory motion must be accounted for daily in order to permit optimum radiotherapy of hepatic malignancies. However, existing tracking systems are often invasive or poorly tolerated by patients. The authors describe the development and validation of an ultrasound-guided tracking and gating system for stereotactic body radiation therapy of the liver.

This noninvasive system is designed to determine the correlation between tumor and external fiducial motion and to verify the optimum gating level for treatment delivery daily. A tracked ultrasound probe moves with patient respiration, obtaining 2D ultrasound images of tumor motion throughout the respiratory cycle. The target volume is registered to the static radiotherapy treatment beams in order to verify optimum gating levels. These gating levels are then transferred to an existing gating system for treatment delivery. The authors examined the temporal and spatial accuracy of this system using a custom-built phantom and verified the accuracy of gating level transfer and delivery.

The temporal accuracy of the ultrasound-guided system was shown to be comparable to the existing clinical x-ray imaging system. Using ultrasound rather than x-rays to image internal targets provides good soft-tissue contrast without the invasiveness of implanting fiducial markers. High frame rates enable continuous monitoring of the target throughout the respiratory cycle. The authors anticipate this passive monitoring system should be well tolerated by patients.

The system developed provides good quality video of the laboratory motion phantom and can be successfully used in gated beam delivery.