The development of dynamic collimation for volume-of-interest cone beam computed tomography and intrafaction motion monitoring during radiation therapy

X-ray imaging in radiotherapy has long been used to localize the target volume prior to treatment. Presently, rotating a low energy diagnostic quality x-ray source around the patient accomplishes this task. A series of images are acquired throughout the rotation. These images are reconstructed to form three-dimensional images of the patient's anatomy, this is known as cone-beam computed tomography (CBCT). The acquired images are then used to correctly position the patient for radiotherapy. However, during the rotation of the x-ray source the radiation field is static. This results in an unnecessarily high imaging dose of radiation to the patient, especially in bone and the surface of the patient. Volume-of-interest (VOI) CBCT involves shaping the radiation field as a function of angle such that each image contains only information necessary to position the patient. Compared to traditional techniques, VOI CBCT is anticipated to increase image quality (by 45%) and decrease the dose inside (by 63%) and outside (by 90%) the VOI. Internal organ motion, such as that observed by the prostate, increases the volume surrounding the target that receives a high dose of radiation. This becomes problematic when using smaller number fractionation techniques with a higher dose per fraction. Monitoring the position of internal target motion during radiation therapy can be used to dynamically track the target. Currently, this is done by using a static imaging field of radiation to image fiducial markers placed inside the target volume. These images are then used to estimate the target location. Similar to CBCT the radiation field an be shaped to one fiducial marker as a function of imaging angle. This will significantly reduce the imaging dose for tracking internal motion during treatment. We intend to explore this application for prostate and breast cancer. At current cancer incidence rates this technology could affect 622800 breast patients and 341620 prostate patients worldwide.