Clinical Application of Ultrasound Imaging in Radiation Therapy

Radiation therapy plays an important role in cancer treatment. It is well known that good local control is achieved when planned dose of radiation is delivered to the target. Recent advances in technology, in particular in image guided radiation therapy (IGRT), has significantly improved the accuracy of target localization for daily radiation treatment. Daily localization of the target is critical to the delivery of the prescription dose to the target. Thus, to achieve accurate targeting and reduce the irradiation of normal tissues and to potentially escalate dose to target volumes, IGRT needs to be implemented for daily use in the clinic. Various IGRT techniques are currently available. One of the techniques integrates an On Board low kilovoltage imaging capability into the linear accelerator that produces diagnostic quality images. However, this imaging technique requires that extra radiation dose be delivered to the patient. Another IGRT imaging system that has been integrated recently with the linear accelerator is 3D ultrasound imaging. This technique is non invasive, requires no extra radiation to a patient and provides capabilities for daily target localization and verification prior to the delivery of radiation treatment. Currently, 3D ultrasound imaging is used for target localization and verification of prostate, gynecological and breast cancers. Since the late nineties, 2D ultrasound imaging has been used for prostate localization only, but only recently has 3D ultrasound localization been available. This has also led to the use of 3D ultrasound for localizing other treatment sites as well; although, each site requires unique methods. For the prostate cancer treatment, the prostate can move daily compared to the reference planning CT (radiation therapy planning and dose calculation for all disease sites is done with CT image based)due to bladder and rectal filling. Ultrasound imaging can be used to image and localize the prostate target daily. The prostate and bladder can all typically be well visualized and compared to the reference ultrasound image or CT image. Shifts are then identified and made to reposition the patient to the point that the treatment volumes identified each day are aligned with where they were on the planning CT images relative to the linear accelerator’s isocenter. This isocenter is the point in space about which the linear accelerator rotates and all planning and radiation beam delivery for a daily treatment is performed relative to it. For gynecological cancer treatment, the ultrasound image does not show the entire target because target definition is complex and composed of multiple structures. The purpose of