Real-time visualization of ionizing radiation for improving quality in radiotherapy [electronic resource]
Keywords: Mechanical Engineering.

Radiation therapy is an integral part of the current treatment regimen for nearly all cancer patients. The success of radiation therapy depends heavily upon the radiation being delivered in the correct amounts to the correct locations at the correct time. Unfortunately, technological limitations currently require that nearly all radiation therapy treatments are performed without an effective feedback mechanism and typically without any in vivo verification. Rather, the field relies heavily upon careful, but separate, control of radiation delivery and patient positioning. As delivery systems and techniques advance in speed and complexity it is vital that new methods for verifying system performance and, if possible, in vivo verification be developed. This dissertation sets forth a method by which high energy x-rays may be visualized using a simple digital camera. Utilizing such a camera enables simultaneous capture of information about x-rays and the subject with which they are interacting. This concurrent capture of information enables a transformative view of radiation therapy beams and enables several critical applications including treatment monitoring and autonomous quality assurance measurements of both mechanical and dosimetric aspects of machine performance. The approach is first demonstrated in a real-time treatment monitoring application. Here, a flexible scintillating sheet is placed on and allowed to conform to the patient. As the radiation beam traverses the sheet, visible light photons are emitted and captured by a nearby digital camera. Ambient room light reflected from the surface of the patient is also captured by the camera. This concurrent capture of information allows a real-time view of both the patient and the impinging radiation beam. This provides a powerful, in context, view of radiation therapy not previously possible. The system is characterized and shown to operate successfully across a variety of room lighting conditions. The second application for which the approach is used is autonomous mechanical quality assurance measurements for linear accelerators. For this application, a system is designed to autonomously perform several measurements used to evaluate the mechanical accuracy of the linear accelerator such as the agreement between the intended and delivered field size and its alignment with various patient alignment systems. The system consists of a radioluminescent phantom and a collimator mounted digital camera. The system includes a self calibration routine that automatically compensates for variation in phantom setup. System generated measurements are compared to measurements from existing, gold standard, techniques and found to be consistent. Measurement uncertainty is shown to be less than 1 mm and independent of phantom setup. The system is able to perform a set of tests normally taking over 1 hour in approximately 10 minutes. Finally, the approach is evaluated for it's potential to perform dose measurements. A new phantom is designed that includes additional optical calibration fiducials. An image processing workflow is developed to compensate for several potential sources of uncertainty in the measurements. The system is then characterized as a detector and shown to have uncertainties of approximately 1\% for output or single point measurements and 3\% for profile, or spatial, measurements. The results show that the system is invariant to room lighting and camera to phantom pose. The detector exhibits a moderate over-response to field size changes, a characteristic common to phosphor-based planar imaging devices. The system also exhibits a dependence on dose rate, though early investigation indicates this may be an artifact of the image processing algorithm. The applications presented in this dissertation demonstrate a radioluminescent phosphor based beam visualization approach for visualizing, monitoring and evaluating radiation therapy beams. This capability may lead to improved quality in radiotherapy by enabling advanced quality assurance measurements as well as increasing the precision and uniformity of quality assurance measurements while decreasing the time and complexity of performing such measurements. By so doing, it may be possible enable safe, high quality radiation therapy delivery in areas of the world where a lack of trained personnel limits the current quality and availability of care. The technique may also lead to new methods for treatment monitoring, in vivo verification and closed-loop delivery.