Feasibility of Using Real-time Cine-MRI for Treating Moving & Deforming Tumors

Accurate dose delivery remains one of the weakest aspects of radiotherapy, especially in the case of thoracic and abdominal tumors, where significant patient motion occurs during dose delivery (intrafraction motion). Such motion results in geometric and dosimetric uncertainties that compromise treatment quality. Effective management of intrafraction motion is therefore key to realizing the full potential of modern image-guided radiation therapy (IGRT). While external markers have been found to be well-correlated with internal anatomy within an imaging session, there is no guarantee that these correlations will continue to exist and be constant throughout the course of the therapy. In general, implanted, radio-opaque seeds have been found to be more reliable than external markers. However, implantation of fiducials, whether radio-opaque or electromagnetic, is necessarily invasive and carries with it the risk of associated complications - an issue that becomes especially important for cancer patients with weakened immune systems. Currently, MR imaging is the only modality that is non-invasive and provides high quality volumetric information for the whole body. The "ideal" intrafraction motion management requires complete spatio-temporal knowledge of the irradiated anatomy. However, to date, there is no clinical method of directly visualizing the tumor volume during dose delivery. Most techniques rely on external or internal surrogate markers which often provide (usually non-volumetric) information of limited accuracy and reliability. In addition, internal markers impose significant "costs" on the patient in terms of interventional complications and increased imaging dose. In this work, we investigate the feasibility of using in-room, fast cine MR imaging as a non-invasive means to provide real-time, soft-tissue-based, volumetric image guidance for continuous monitoring of the target and surrounding anatomy. To date, there has been no systematic investigation of the imaging requirements of an integrated MRI+linac for the specific task of real-time radiotherapy guidance.