Feasibility Study of Real Time Ultrasound Intrafractional Monitoring in Liver Stereotactic Body Radiation Therapy Underactive Breathing Control

Abstract

Stereotactic body radiation therapy (SBRT) allows delivery of high doses to liver tumor with steep dose gradients to surrounding normal tissue and thus limits risk of liver toxicity. Intra-fraction target motion remains one of the major sources of uncertainty in liver SBRT. It blurs the planned dose distribution and thus undermines the purpose of SBRT. Ultrasound is a cost effective and non-ionizing approach for motion monitoring. In this study, we investigated the feasibility of an existing ultrasound prostate monitoring system for liver SBRT under active breathing control (ABC) and its monitoring accuracy in simulated phantom experiment. An abdominal ultrasound phantom with six tumors inside the liver was used. The phantom was secured on a programmable respiratory motion platform. A 4D probe was modified for transabdominal imaging with flexible infrared tracking angle. A medical grade arm and bridge secure system (ABS) was developed to hold ultrasound probe. The motion platform was programmed to perform repeated mechanical movement to simulate ABC patient motion. Platform was programmed to move in superior-inferior direction, which is the major tumor motion direction observed in our patients. The movement step was set to be 0 mm with 20 seconds followed by 10 mm in inferior direction with 20 seconds in one cycle to simulate the expiration and inspiration breath hold phases of the ABC. In total of 10 cycles of movement were repeated and the expiration ultrasound was taken as the reference image. 4D ultrasound images were then acquired continuously for monitoring. For the 10 cycles of movements of 10 mm in inferior direction, the average real-time movement distance reported by 4D motion monitoring system was 9.92±0.13 mm and with maximum deviation less than 0.3 mm compared with programmed step. No probe fixation alert was detected during the whole experiment by the infrared tracking system. Our 4D motion monitoring system was able to accurately recover the liver tumor motion in our phantom study. Our ABS system is able to hold the probe steady during the whole motion simulation. This suggests with modification, current 4D prostate ultrasound system can be applied to real-time intra-fractional motion monitoring in liver SBRT with ABC.