The clinical rationale for MRI-guided radiotherapy : the dawn of a new era

Abstract

Current radiotherapy treatment machines use the treatment beam or the integrated cone-beam computed tomography (CBCT) functionality for patient positioning based on bony structures or implanted tumour markers. The University Medical Center Utrecht, in cooperation with Elekta and Philips, is developing a radiotherapy accelerator with fully integrated diagnostic magnetic resonance imaging (MRI) functionality. MRI provides superior soft-tissue contrast, which will enable patient positioning based on the tumour itself. The clinical rationale for the MRI-accelerator system is considered in this thesis. The rationale is evaluated for various tumour sites on different time scales. The benefit of this unique hybrid design can be divided into two aspects: online image guidance and dose response assessment. This thesis is limited to the aspect of online image guidance. The fact that most soft tissue structures not only change in position but also in shape makes it inevitable to use online volumetric image guidance for improved daily verification of the patient set-up. If the image guidance is fast enough, real-time monitoring or tracking of highly mobile tumours will become feasible, making radiotherapy a treatment option for more tumour sites. Cervical cancer patients will benefit in terms of healthy tissue sparing in the primary tumour region as the target volumes change their position and shape continuously during the course of the treatment. Online MRI images, acquired at the beginning of each treatment fraction, visualise the actual position and shape of the uterus and can be used for a simple rigid correction or a treatment plan adaptation. This approach allows planning target volume (PTV) margin reduction as interfraction motion is corrected directly on the treatment table. Guidelines to limit intrafraction bladder filling may further reduce the needed PTV margins, which results in additional healthy tissue sparing. Our current clinical practice of marker based position verification for prostate cancer patients is fairly accurate. Therefore, the benefit of MRI-guided radiotherapy for prostate cancer patients will be the daily visualisation of the healthy surroundings. This allows a better complication risk assessment and a safer dose escalation to the intraprostatic lesions located close to the rectal wall. Surgery is the only established curative treatment option for renal cell carcinoma. Urological complications as intraoperative hemorrhage and postoperative urine leakage still occur; therefore a non-invasive approach remains the ultimate treatment. The technical feasibility of real-time MRI-guided radiotherapy in terms of breath-holding, treatment planning, and radiation delivery time is presented. The clinical rationale for the MRI-accelerator system is not limited to the tumour sites described in this thesis. The MRI-accelerator has great potential for all tumour sites that exhibit large variation in position, size, or shape during the course of the radiotherapy treatment, for example the bladder, the rectum, the pancreas, and the liver. The clinical introduction of the MRI-accelerator will be facilitated by the construction of a dedicated center for image guided oncological interventions, which will house three MRI-accelerators. Improvement of current radiotherapy treatments as well as the development of radiotherapy strategies for new tumour sites will be investigated.