Radiation therapy (RT) is a core pillar of oncologic treatment and half of all patients with cancer receive RT, as curative or palliative treatment. Recent integration of magnetic resonance imaging (MRI) into the RT workflow, led to the advent of MRI-guided RT (MRIgRT). Using MRI as the imaging technique rather than computed tomography has clear advantages for guiding RT to pelvic tumours: superior soft-tissue contrast, improved organ motion visualization, and the potential ability to image tumor phenotypic characteristics to identify the most aggressive or treatment resistant areas which can be targeted with a more focal higher RT dose. Radiologists should be familiar with the potential uses of MRI in planning pelvic RT, the varied RT techniques used such as brachytherapy and external beam radiotherapy and the impact of MRIgRT on treatment paradigms.The current clinical experience and evidence base for MRIgRT in prostate, cervical and bladder cancer will be discussed, and examples of treated cases will be illustrated. The benefits of MRIgRT such as real-time online adaptation of RT (during treatment) and interfraction and/or intra-fraction adaptation to organ motion will be highlighted along with how MRIgRT can improve toxicity and oncologic outcomes. MRIgRT is particularly beneficial for treating mobile pelvic structures and real-time adaptive RT treatment for tumors can be achieved using novel MRI-guided linear accelerator (MRI-LINAC/ MRL) systems in order to spare organs at risk. Future opportunities for the development of biologically driven adapted radiotherapy (ART) using functional MRI sequences and radiogenomic approaches will be outlined.
Summary StatementMagnetic resonance imaging guided radiation therapy (MRIgRT) for treating pelvic tumours offers superior soft-tissue contrast, improved organ motion visualization, and the potential ability to image tumor phenotypic characteristics with real-time online adaptation of treatment plans to potentially improve toxicity and oncologic outcomes.
Teaching points [marked in main text]• Currently CT is used throughout the RT pathway however using MRI as the imaging modality has clear advantages: Superior soft-tissue contrast allows for more accurate delineation of the tumor target and organs at risk, improved organ motion visualization, and the ability to image tumor phenotype characteristics using quantitative multiparametric MRI (mpMRI) sequences• The availability of hybrid systems incorporating MRI with a linear accelerator (MR-LINAC/ MRL) offers the opportunity for real-time MRI at each RT fraction, allowing for modification of patient position and treatment plan while the patient is on the treatment table• Each RT fraction starts with acquisition of an online MRI which is used to perform RT plan adaptation through two different workflows, either adapt to position (ATP) or adapt to shape (ATS)• The ATS workflow is used to treat structures that are mobile and may change shape e.g.
bladder and cervix• The ATP approach is used to treat prostate tumours where the...