The role of radiotherapy in the management of primary and metastatic liver malignancies has expanded in recent years due to advances such as IGRT and SBRT. MRI-guided radiotherapy (MRgRT) has arisen as an excellent option for the management of hepatocellular carcinoma, cholangiocarcinoma, and liver metastases due to the ability to combine improved hepatic imaging with conformal treatment planning paradigms like adaptive radiotherapy and advanced motion management techniques. Herein we review the data for MRgRT for liver malignancies, as well as describe workflow and technical considerations for the 2 commercially available MRgRT delivery platforms.

Since the introduction of on-board magnetic resonance image (MRI) guidance to radiation therapy (MRgRT) clinical practice in 2014, this innovative technology has been shaping a new treatment paradigm for the management of both primary and secondary liver malignancies. Historically, local control via surgical resection has been the only curative intervention, provided there were no limiting comorbidities and sufficient liver reserve, though unfortunately only a fraction of patients with liver

With several years of clinical use, there is mounting data supporting the use of on-board MRgRT in multiple disease sites. Much of the early literature evaluated high patient-volume sites such as lung, prostate, and breast, though arguably upper GI malignancies would gain the most benefits from MRgRT. On-board imaging systems such as cone-beam CT (CBCT) are ill-sutied to imaging upper GI malignancies because of their long image acquisition times and their reliance on x-ray linear attenuation

The 0.35T MRgRT system workflow for liver SBRT will now be described in detail. The treatment planning process begins with a MRI screening questionnaire followed by 4DCT and MRI simulation with the patient in the treatment position using the clinic's immobilization hardware. MRI coils are placed on the patient's abdomen or outside the immobilization device.A CT simulation is typically also acquired to both contour tumors and OARs and provide the electron density distribution for dose

Prior to simulation, liver patients undergo a screening process to evaluate their compliance with active breathhold control (ABC) and abdominal compression techniques. Typically, only patients who are not able to comply with ABC are selected for the 1.5T system treatments, as there is not a commercially available motion management system (at the time of writing this manuscript). The patient setup consists of arms placed in a down position and a pressure belt that is adjusted to minimize organ

Now that we have described the workflows for the 2 primary MRgRT systems on the market, we will described technical imaging considerations for these machines. Figure 3 demonstrates images of the normal liver from a diagnostic MRI as well as the baseline sequences for the 0.35T and 1.5T systems. The standard images acquired on the 0.35T system is a True Fast Imaging (TRUFI) MRI sequence with steady state free proecession. TRUFI provides image contrast that is a combination of a T1 and T2

MRgRT has improved the delivery of radiotherapy for patient's with primary and metastatic liver maliagnanices. Through improved on-board imaging and innovations such as online adaptive re-planning and real-time MR-cine gating, MR-guided delivery of radiotherapy has improved the dosimetric therapeutic ratio of liver-directed radiotherapy, with early data demonstrating that this may translate to a clinical benefit for these patient populations. While there are significant workflow and technical

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