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

Breathing gating systems can help increase tumour control probability and reduce normal tissue toxicity during stereotactic body radiotherapy of liver metastases. Using an ultrasound machine, a video processing computer and an existing infrared camera system, we developed an ultrasound-guided tracking and gating system. The system reconstructs tumour motion in room coordinates and facilitates the determination of gating levels based on tumour motion. The delay of the imaging system was investigated at various imaging settings and conditions with an in-house built motion phantom. We compared our system's delay to that of the currently existing kV x-ray imaging at various tumour motion speeds and found the two systems equivalent. Typical delays varied around 10+/-15 ms for both systems that translated into very good positional accuracy in imaging (<1 mm error). Gated beam delivery, based on gating levels determined with our system, also provided good results.