Hybrid magnetic resonance (MR)-guided linear accelerators represent a new horizon in the field of radiation oncology. By harnessing the favorable combination of on-board MR-imaging with the possibility to daily recalculate the treatment plan based on real-time anatomy, the accuracy in target and organs-at-risk identification is expected to be improved, with the aim to provide the best tailored treatment. To date, two main MR-linac hybrid machines are available, Elekta Unity and Viewray MRIdian. Of note, compared to conventional linacs, these devices raise practical issues due to the positioning phase for the need to include the coil in the immobilization procedure and in order to perform the best reproducible positioning, also in light of the potentially longer treatment time. Given the relative novelty of this technology, there are few literature data regarding the procedures and the workflows for patient positioning and immobilization for MR-guided daily adaptive radiotherapy. In the present narrative review, we resume the currently available literature and provide an overview of the positioning and setup procedures for all the anatomical districts for hybrid MR-linac systems.
However, these advanced online re-planning solutions are still burdened by longer treatment times, which may affect intra-fractional patient and especially organ motion due to the fraction duration. Patient positioning is indeed significantly different from conventional linac treatments due to the small gantry size and the need to include MRI-coils in the immobilization process. In this scenario, the need for a reliable and comfortable patient positioning is a critical feature to perform a safe and effective MR-guided treatment [7, 8].
planning and positioning in mri pdf download
Given the relatively recent commercial availability of the MRI-guided systems, details on patient positioning and immobilization devices are still lacking. The purpose of this narrative review is to outline the currently available literature regarding patient setup in online MR-guided radiation therapy (oMRgRT). In addition, the authors have included their own initial clinical experience from centers equipped with the aforementioned two different systems. In order to provide the reader with a practical reference tool, all positioning devices used at the different institutions are illustrated with pictures at the end of all chapters and reported for the different anatomic regions. In addition, the Additional file 1 file provides tables of the equipment used.
Moreover, as reported by Barnes et al., the absence of lateral lasers in the Unity system and the close proximity to the coils in the MRIdian system mean that minute adjustments to patient positioning to align to lasers are not possible. Rather patient positioning is focused on general patient comfort and positioning and may be as effective [9].
With respect to positioning devices, MRI safety aspects are obviously of paramount importance. In addition to adequate patient screening for MR-compatibility, all equipment must be designed and tested for a dedicated use in a MR environment. The equipment must be approved for ferromagnetic safety, which is usually already confirmed by the manufacturer. However, it is recommended that the safety status of each device is verified as part of an on-site QA process to test ferromagnetic properties, imaging artefacts, dose attenuation level and physical compatibility with the coils.
All MR conditional equipment should be labeled to avoid mix-ups with non-MR compatible positioning devices. However, most manufactures already use designated colors to avoid this sort of complication. Other issues to be considered for patient positioning are the limited gantry size (70 cm), which imposes additional restrictions compared to conventional radiotherapy patient positioning. In particular, for obese patients or when patients are positioned with both arms above the head, the remaining space to the bore wall might be very small. In some cases, this configuration may not be possible at all and patients need to be positioned with their arms parallel to the body. An additional consideration is the approach taken when there is machine breakdown. If the patient is to be treated on a conventional linac, the immobilization systems must be transferrable and staff familiar with them.
Another point to consider is that the MR-specific equipment must be integrated into patient positioning. In addition to the already mentioned coils, patients need hearing protection (earmuffs and/or earplugs) and an emergency squeeze bulb or push-button alarm. Moreover, the patients need to be instructed on how staff will communicate with them during treatment delivery. Overall, because of the longer treatment times in adaptive MRgRT, the patient set-up should be as comfortable as possible to increase compliance and reduce intrafraction patient movement and potential claustrophobic reactions. For this reason, in some centers the use of prism glasses and a TV screen outside the bore are applied to make more pleasant the stay in the treating room. Of note, this device cannot be applied for head and neck treatments. (see Additional file 1).
For radiotherapy treatments of the brain and head-and-neck region, the use of thermoplastic masks remains the gold standard to prevent motion of the head and guarantee reproducible patient positioning [17].
In the particular environment of MRgRT, it is principally challenging to perform patient immobilization that includes proper coil positioning and hearing protection in addition to the thermoplastic mask. To date, many institutions have created their own in-house developments to allow proper coil placement. However, there are already some dedicated systems commercially available (see Additional file 1: Table 1).
To date, there is limited evidence available from preliminary reports of MRgRT treatments for brain malignancies [18]. While there are some experiences using radiotherapy positioning devices in diagnostic MRI scanners to obtain diagnostic imaging in treatment position [19], evidence from hybrid MR-linac systems is scarce. Moreover, none of the available hybrid systems provides specific brain coils for dedicated imaging.
Examples of the systems used at the contributing institutions can be found in Fig. 2. Figure 2 (A) shows an example of the immobilization used for brain irradiation with the MRIdian system. In this case the so-called head and neck coils are used. The posterior surface receive coils (flat without plastic bar) are positioned on the table and the HeadSTEP UP VR system (IT-V, Innsbruck, Austria) is placed on top of the coil and fixed on the table using appropriate indexing bars. The patient`s head is positioned with appropriate MR-compatible pillows and fixated with a custom made thermoplastic mask (IT-V, Innsbruck, Austria). Then the anterior receive coil is positioned at the top and hooked into the HS Flexcoil holder VR of the HeadSTEP system in order to avoid touching of the patient`s face. The setup for head and neck MRgRT is similar and an example is shown in Fig. 2 (B). Since the field of view for MR imaging must be further inferior in head and neck irradiation than for cerebral RT, the torso coil is used as posterior coil and a dedicated HeadSTEP UP VR H&N system is mounted above it, with a longer flexi-coil holder for adequate positioning of the anterior receive H&N coil.
Examples of patient positioning for a brain and b head&neck radiotherapy using the MRIdian system (Viewray Inc., Cleveland, USA) and c, d using the Elekta system (Elekta AB, Stockholm, Sweden)
Figure 2 (C) and (D) shows examples of patient positioning using the Unity system. Patient immobilization is performed in supine position with the arms along the body. The customized thermoplastic mask (IT-V, Innsbruck, Austria) is mounted to the indexed HeadSTEP MR system (Elekta, Stockholm, Sweden), with the head of the patient positioned on a MR-compatible pillow. Then the coil is positioned and fixed to the table.
A recent position paper by Koerkamp et al. [39] has outlined the main problems of patient positioning for MR-guided breast radiotherapy. In particular, both supine and prone positions present practical challenges as the coils must be included in the positioning process without compromising the whole body contour, which is necessary for treatment planning. In addition, organ motion must also be considered; although Ahn et al. [40] reported that prone position is the optimal choice for minimizing thoracic respiratory motion and consequently artefacts generation; the study by Batulamai et al. observed no relevant impact of patient position on image quality and motion artefacts, whether in prone or supine position [41].
Fischer-Valuck et al. [44] described their experience in the treatment of breast cancer, accounting for 26% of the cases treated with the MRIdian system in the first 2.5 years of activity. Unfortunately they did not provide any detailed information regarding patient positioning in the article. Nachbar et al. [45] reported the first case of partial breast irradiation (PBI) treated with a 1.5 T MR-linac. Both planning CT and MRI were performed in supine position with the use of a positioning device in free breathing. The article also highlights the electron air stream effect (ESE), which can lead to out-of-field dose deposition and the electron return effect (ERE), which may result in increased dose to the skin and at air/tissue interface. Especially in breast RT, where the target volume directly involves the skin, these effects can cause an increase dose to the skin and also out-of-field skin dose on the chin. Thorough plan optimization and bolus placement on the chin are emphasized. This effect is less pronounced in 0.35 T systems [46].
Examples of patient positioning for breast radiotherapy using the a MRIdian (Viewray Inc., Cleveland, USA) and b Elekta system (Elekta AB, Stockholm, Sweden) with c Bolus placement 2ff7e9595c
Commentaires