Managing motion in MRI-guided liver radiation therapy

The liver is a common location for primary cancer and metastatic disease. Patients are frequently treated with radiotherapy if they are contraindicated for surgery. Stereotactic body radiation therapy (SBRT) is a method to deliver a highly conformal radiation dose to the target with a rapid dose drop-off in a small number of treatment fractions, resulting in a high biologically effective dose. The success of liver SBRT critically depends on precise dose delivery to the tumor, which is challenging due to respiratory-induced organ motion and inter- and intra-fraction position changes. Poor liver tumor contrast on conventional X-ray based in-room imaging requires the implantation of fiducial markers to guide the treatment. This is an invasive procedure with the risk of complications and surrogacy errors. Magnetic resonance imaging (MRI) provides excellent soft-tissue contrast. Therefore, integrated MRI and radiotherapy systems such as the MRIdian (ViewRay Inc., Cleveland, OH, USA) and Unity (Elekta AB, Stockholm, Sweden) MR-Linac have the potential to visualize moving lesions in the upper abdomen directly, eliminating the invasive marker procedure. In this way, it might be possible to decrease treatment margins in the future, increasing the accuracy of image-guided liver SBRT. In the past, different motion management strategies have been developed for liver SBRT. Irradiation of the tumor in its time-weighted averaged position (midP), provides an efficient free-breathing treatment. To enable a non-invasive free-breathing treatment, optimizing patient comfort and treatment time, and potentially increasing treatment accuracy, MRI-based solutions to facilitate mid-position liver SBRT on the MR-Linac are developed and validated in this thesis.