Chapter 1 18 motion can significantly impact the dose delivered to the target as well as the OARs, thereby potentially increasing the risk of toxicity and lowering the effectiveness of treatment.63 With fewer fractions, there is less room for delivery errors to be blurred over the course of treatment. Therefore, optimal image-guidance before and during treatment as well as intrafraction adaptation possibilities are desired. MRI-guided radiotherapy Over the past decades, image-guided radiotherapy (IGRT) has evolved immensely. Technological developments have eventually led to the clinical introduction of MRI-guided radiotherapy using MRI-guided linear accelerator systems (MR-Linac). Currently, two systems are on the market: the 0.35 Tesla (T) MRIdian (ViewRay Inc., Oakwood, U.S.A.) and the 1.5 T Unity (Elekta AB, Stockholm, Sweden). The idea for integrating a linac with a 1.5 T MRI scanner originated in Utrecht.64–66 The 1.5 T MR-Linac system was developed at the University Medical Centre Utrecht (UMC Utrecht, Utrecht, The Netherlands), together with Philips (Best, The Netherlands) and Elekta AB (Stockholm, Sweden). It consists of a 1.5 T Philips MRI scanner surrounded by a 7 megavolt (MV) linac, mounted on a ringshaped gantry. The Unity MR-Linac provides diagnostic quality MR imaging before and during treatment. In May 2017, the first patient was treated on the system.67 In Utrecht, the first prostate cancer patient was treated on an MR-Linac in February 2019 with 20 fractions of 3.1 Gy.62 The greatest benefit of MRI-guided linacs over CT-guided linacs is the superior soft-tissue contrast, allowing clear visualisation of the target and OARs.59 Because of the superior contrast, no fiducials are needed to verify the position of the prostate. Also, tattoos for patient positioning are no longer required with the daily plan adaptation. Additionally, continuous 3D imaging is possible and no ionising radiation is used to obtain MR images, allowing unlimited imaging without additional harm to the patient.61,62 Treatment preparation for MRI-guided SBRT Prior to the actual dose delivery (online phase), the treatment is prepared (offline phase) by acquiring pre-treatment imaging of the target area, generally consisting of CT and/or mp-MRI scans. For most prostate cancer patients at the UMC Utrecht, an MRI-only workflow is applied, i.e. without obtaining a planning CT scan but instead using a pseudo-CT scan created from dedicated MR images.68,69 The treating physician identifies and delineates the target and relevant OAR structures on the MRI scan (Figure 3). For the treatment of prostate cancer, the Gross Tumour Volume (GTV) is delineated. The GTV encompasses the visible tumour on mp-MRI. To cover any potential microscopic tumour spread, the GTV is enlarged to create the Clinical Target Volume (CTV). The CTV includes the prostate body and the GTV with a 4 mm margin, excluding the bladder and rectum. Depending on the location of the tumour and the risk classification, the seminal vesicles are (partly) included in the CTV. To account for geometric inaccuracies and uncertainties, including machine uncertainties and patient set-up errors, the CTV is enlarged by a 5 mm isotropic margin to create the Planning Target Volume (PTV). In addition to the target, the bladder, rectum, and several bony structures (e.g. femur heads) are delineated and identified as OAR. Using the pre-treatment imaging and delineations, a baseline radiotherapy treatment plan is created. The treatment plan is optimised to ensure adequate target coverage without violating the dose constraints for the OARs.
RkJQdWJsaXNoZXIy MTk4NDMw