Ultrasound Monitoring of Abdominal Soft Tissue

Within the last decade, Linac based stereotactic body radiation therapy (SBRT) has been shown to be an effective treatment option for pancreas1, 2 and liver3, 4 tumors. SBRT delivers high doses of radiation therapy to the tumor over only 1-5 treatments. Because of the spatial precision of SBRT, it is feasible to administer a high radiation dose in only a few treatments. By minimizing the amount of radiation to surrounding healthy tissue, it is possible to decrease the rate of toxicity/complication and increase the radiation dose to cancerous tissue, thereby allowing better local control. SBRT of the abdomen has been limited by the movement of intra-abdominal organs that naturally occurs with respiration and bowel movement. Organ motion occurs both intra- and inter-fractionally. While intra-fractional motion is a result of respiration, peristalsis and cardiac motion, the magnitude of inter-fractional target motion is dependent of daily variations in organ filling, weight change, tumor growth and radiation induced changes of tissue. Tumor movement may lead to tumor displacement and suboptimal dose delivery. Accurate localization of the target is very important to improve treatment delivery accuracy and reduce toxicity of the treatment. To evaluate tumor motion due to breathing motion, a four dimensional (4D) CT simulation scan is performed. If the tumor moves more than 3 mm during a breathing cycle, breathing motion management is employed using Active Breathing Control (ABC) technique. ABC requires the patient to hold his/her breath within the proper tidal volume while treatment is delivered, while free breathing may be resumed between periods of treatment. This technique limits the delivery of RT to specific phases of the respiratory cycle so as to minimize the influence of breathing on the delineated tumor. Despite significant progress made in ABC technique, tumor and organ motion could only be minimized and not eliminated completely with this technique. Assessing patient specific tumor/organ motion (both intra- and interfractional) throughout the course of SBRT treatment offers the possibility of ensuring delivery of the prescribed target dose while simultaneously minimizing normal tissue damage. In order to monitor the abdominal soft tissue motion, the Department of Radiation Oncology has developed a 4D ultrasound technique based on an ultrasound probe holder and a continuous motion monitoring software. The 4D ultrasound image is acquired by using a motorized 3D ultrasound probe and image continuously. 4D ultrasound is a new non-ionizing and non-invasive imaging technique that continuously monitors the tumor motion during the radiation treatment in real time.