Session Item

Despite this central role in cancer treatment, today’s radiation therapy treatments suffer from a major problem: whilst they routinely image patients prior to treatment, no anatomical information is typically available during treatment. Tumours are not static during treatment, so methods to monitor tumour motion during radiation targeting are essential to ensure prescribed dose coverage. This is even more critical with high-dose treatments such as stereotactic body radiotherapy (SABR) where a large radiation dose must be delivered in a small number of treatment sessions.

To monitor the target motion in real-time, X-ray imaging are often utilised, including add-on X-ray imaging systems to the treatment room such as the ExacTrac system (BrainLab). The Real-time Tracking Radiotherapy (RTRT) system was one of the room-mounted system to be developed and deployed clinically. Room-mounted dual X-ray imaging systems have deployed for dedicated linear accelerators systems such as the Vero or CyberKnife, the latter of which uses X-ray imaging in conjunction with an optical system for target motion monitoring.

The standard linear accelerator is equipped with one gantry-mounted kilovoltage X-ray imager, which can be used to monitor the target motion. In this case, at least some predictive or probabilistic algorithm is required estimate the 3D motion as each image only contain 2D information of the target. Typically, the inter-dimensional correlation of the patient 3D motion is utilised as a priori for these algorithms, particularly for respiratory-related motion. Motion monitoring using gantry-mounted kV imager was shown to achieved sub-mm accuracy and used clinically at a few centres around the world for prostate cancer treatment and recently, liver SBRT, using implanted fiducial markers as surrogates to the targets. In the future, markerless kV monitoring, particularly for high contrast targets such as lung tumour and the spine, is highly promising.

Finally, the MV imager can be used to capture target motion information in the Beam’s eye view, which is highly advantageous as there is no additional imaging dose is required, the images containing the target information are created by the MV beam. However, MLC occlusion and modulation, together with low imaging contrast in the MV images are technical challenges that need to be overcome to obtain high motion monitoring accuracy.