Benefits of the Newly-Introduced Linac-MR System

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With MRIs’ soft-tissue contrast capability and the need to better target cancerous tumors while being treated by a linear accelerator (linac), the utilization of MRIs in Radiation Oncology has proven to be of great value. Nevertheless, being governed by the basic laws of physics, Linear Accelerators (Linacs) and Magnetic Resonance Imaging (MRI) systems do not work well when placed in proximity to each other [2]. With the availability of several types of MRIs and the different possible geometries, the choice of the MRI and how it is engineered into the Linac-MR system can significantly impact the system’s overall effectiveness and functionality.

Research Question

In designing its medical Linear Accelerator-MRI Hybrid (Linac-MR), what clinical, ergonomic, and financial benefits did MagnetTx’s choice of an open-air bi-planer high-temperature superconducting MRI oriented parallel to the Linac beam line provide when compared to the two currently existing Linac-MR systems?

It is vital to compare and contrast a specific newly introduced Linac-MR system by MagnetTx, the choices this manufacturer has made to the two existing systems commercially available, and its benefits related to the Electron Return Effect, larger treatment fields, and the elimination of the need for MRI quenching. When designing its Linac-MR, the MagnetTx choice of the type of MRI and its orientation significantly impacted the benefits of its Linac-MR system, the Aurora RT. Because of the Lorentz force, charged particles undergo curved trajectories in magnetic fields. The magnitude of Lorentz force depends on several items, including the incident beam’s orientation relative to the magnetic field^,which leads to several clinical issues, and the parallel alignment of the Aurora RT creates several clinical benefits [3]. There is the reduction of the electron return effect and electron streaming effect and open air and larger bore size, allowing for larger treatment fields and reducing claustrophobia.

The open-air MRI, also referred to as LRBP (Longitudinal Rotating BiPlaner), does not need Helium for cooling [10]. With no Helium, there is no need for a quench pipe, reducing the cost of siting the system and lowering the operating costs [10]. Nevertheless, there is the counterargument that loss of skin sparing could emanate from directing electrons along the central magnetic axis. When exceeding the maximum recommended dose, an inline configuration can cause a rise in the entrance surface dose [7]. However, the benefits of the Aurora-RT outweigh its costs.

To begin with, MagnetTx’s option offers reduction of the electron return effect and electron streaming effect [8]. This results in several benefits, including no field shift and penumbra asymmetry, no build-up depth reduction, no electron return effect that causes lung dose, chest wall and skin dose increase, the presence of electron focusing and weaker effects on detector response and less pronounced air gap effects [9].

Moreover, the Aurora-RT offers open-air and larger bore sizes, allowing the ability to treat additional, more significant tumor sites. According to [4], limited MRI bore size poses a significant challenge for patient setup in breast RT treatment positions for hybrid machines or in a magnetic resonance imaging (MRI) scanner. For instance, the inclined position, in combination with the elbow span in patients being treated in a supine position with hands elevated above their head, causes problems [4]. However, the authors note that clinicians could solve this by using no wedge at all, using a wedge with lesser inclination or by putting the arms close together. Another problem that could arise is patients in a supine position getting deformed breasts from an anterior receiver coil. Nonetheless, doctors can prevent deformation by supporting the coil with coil bridges.

The prone position also comes with challenges for smaller bore sizes. A perfect illustration would be how the position needs an extra receiver coil to support the patient’s back and additional space for a pendulous breast to dangle easily without reaching the operating base, limiting the ratio of casualties who can fit into the MR scanner [4]. [3] further back the idea that small bore sizes are problematic by suggesting that one has to find equipment that fits inside the MR bore and leaves enough space for the MR receiver coils. The biggest difficulty in setting up a patient in the treatment position for breast RT in an MRI scanner or a hybrid device is the smaller MRI bore size (60–70 cm) compared to the CT bore size (80–90 cm) (16–18). This reduces the range of viable patient setup positions as well as the size and inclination of a positioning device. However, standard MR equipment like the dedicated prone breast coil is not designed for system reproducibility, and typical RT control apparatus may not necessarily be compatible with the MR [3]. However, the Aurora-RT solves these problems. While usual bore sizes range from 60–70cm bore size for hybrid machines and magnetic resonance imaging (MRI) scanners, the CT offers 80 to 90 cm, increasing the amount of possible positions for patient setup and the inclination of a positioning device [4].

Other problems associated with smaller bore sizes are summarized in the table below, showing how the Aurora-RT has gone a long way in improving simulation, contouring and planning due to its larger bore size:

Figure 1: Problems solved by the Aurora-RT’s bore size

Challenge	Effect	Potential Solution
Contouring Surgical clip and/or marker visualization on MRI	Artifacts impeding contouring of target volume and distortion of magnetic field.	1. Utilize clips or markers with smaller artefacts. 2. No marker insertion (only possible in the neoadjuvant setting if no further surgery is required)
Simulation and Planning Geometric accuracy (gradient nonlinearities) in combination with lateral target volumes. Geometric accuracy (magnetic field inhomogeneities and patient-induced distortions)	Reduced geometric accuracy, increasing with distance from isocenter Reduced geometric accuracy, particularly near tissue–air Interfaces.	1. Use distortion correction software on the scanner 2. Position the target as close to the scanner isocenter as possible (e.g., shift patient on the table) 3. Include remaining inaccuracy in the PTV margin 1. Use high bandwidth acquisition 2. Acquisition of B0 map to assess patient-induced distortion.
Planning Electron return effect Electron stream effect Missing electron density information in MR-only workflow High-density treatment couch material	Possible lung, chest wall or skin dose increase (dose increase at the tissue–air interfaces) Irradiation dose outside the treatment field in an inferior-to-superior direction Inaccurate dose calculation without correct electron density assignments. Unpredictable dose effects by daily re-planning.	Pay attention to skin, chest wall, and lung dose constraints in planning, and carefully choose beam setup (e.g., use enough beams) Use of bolus material to shield irradiation outside of the field. Development of methods for synthetic CT generation from MRI. Avoid beam angles passing through the treatment couch edges.
Treatment Irradiation through coil Fixed treatment couch Motion during treatment	No irradiation through MR receiver coils, only through dedicated hybrid machine coils. A dedicated prone breast coil cannot be used. Interfractional changes in position cannot be corrected by moving the treatment couch. Geographical miss during treatment or increased PTV margins	1. Try to fit the dedicated MR-linac coil on top of prone patient (only for smaller patients). 2. Design a thinner, more flexible coil for the hybrid system. 3. Design a new prone coil for the hybrid system. Use online plan adaptation strategies to account for interfractional changes in anatomy. Use online gating or tracking when available, e.g., only beam-on when the target volume is within pre-specified boundaries.

With open air and larger bore size, claustrophobia is also reduced. [6] define claustrophobia as the fear of enclosed spaces to the extent of interference with daily activities such as school and work. This cannot be the case with the Aurora-RT, as it is an open machine. In a study conducted by [5] to assess the superiority of wide and open MR scanners in alleviating claustrophobia, findings suggest that symptoms of claustrophobia such as ringing sound in the ears, tingling or numbness, dry mouth, feeling light-headed, faint and dizzy, feeling butterflies in the stomach or stomach upsets, choking feeling, chills, breathing fast or trouble breathing, rapid heartbeat or tightness in the chest, shaking, sweating, clinging, freezing, tantrums and crying are reduced by 90%. Emotional symptoms such as the fear of death, an intense urge to leave the situation, overwhelming anxiety, dread, and the fear of losing control are also reduced by the same ratio [5]. Since the Aurora-RT’s larger bore size provides more space than a fixed MRI and is open, it ensures that the head remains outside during scanning, eliminating the fear of enclosed spaces. However, claustrophobic patients may still be slightly anxious, as [1] suggest.

Finally, the high-temperature superconducting magnet eliminates the need for liquid Helium and reduces operating and construction costs. As presented by [10], being a 0.55T MRI, the Aurora-RT offers a 40–50% savings potential compared to a 1.5T MRI system in terms of the purchase price. If the operating spot is at ground level, the exterior façade of the Aurora-RT does not have to be opened, as its small size permits setting up using a remotely controlled portable robotic system. When combined with the absence of necessity to fit a quench pipe, the total installation cost reduces by 70%. Possibly lesser energy intake for cooling and examinations, smaller room size, and lower maintenance costs result in further cost reductions [10]. Therefore, the use of lower-field strength MRI systems offers huge environmental and economic potential for both infirmaries and clinicians, and the healthcare system as a whole.

Nevertheless, there is the counterargument that an inline configuration can cause a rise in the entrance surface dose, as suggested by [7]. The author describes that “a high field inline configuration does give rise to a high surface dose due to electron focusing along the central magnetic axis that can exceed the dose at max leading to a loss of skin sparing” (p. 4). This can be remedied by paying attention to skin dose constraints in planning and carefully choosing a beam setup [2]. This renders the benefits of Aurora-RT superior.

In conclusion, the MRI and engineering decisions in designing the Aurora-RT offer significant clinical and financial benefits over the other two clinically available Linac-MR systems. As discussed above, MagnetTx’s Aurora-RT allows for the reduction of the electron return effect & electron streaming effect, resulting in several benefits, including no field shift and penumbra asymmetry, no build-up depth reduction, no electron return effect, the presence of electron focusing and weaker effects on detector response and less pronounced air gap effects. Its open-air and larger bore size also allow the ability to treat additional, more significant tumor sites. Usual sizes range from 80 to 90 cm for the CT and 60–70cm bore size for hybrid machines and magnetic resonance imaging (MRI) scanners, limiting the amount of possible positions for patient setup and the inclination of a positioning device. With open air and larger bore size, claustrophobia is also reduced. Finally, the high-temperature superconducting magnet eliminates the need for liquid Helium and reduces operating and construction costs. Being a 0.55T MRI, the Aurora-RT offers 40–50% savings potential compared to a 1.5T MRI system in terms of the purchase price. If the working spot is at ground level, the exterior façade of the Aurora-RT does not have to be opened, as its small size allows for setting up using a remotely controlled portable robotic system. When combined with the lack of necessity to fit a quench pipe, the total installation cost reduces by 70%. Possibly lesser energy intake for cooling and examinations, smaller room size, and lower maintenance costs result in further cost reductions. Despite the counterargument that loss of skin sparing could emanate from directing electrons along the central magnetic axis, the Aurora-RT offers numerous advantages that outweigh its disadvantages. Loss of skin-sparing could be averted by paying attention to skin dose constraints in planning and carefully choosing an appropriate beam setup.

References

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[3] Kirkby, C., Rathee, S., & Fallone, B. G. (2010). Lung dosimetry in a linac-MRI radiotherapy unit with a longitudinal magnetic field, American Association of Physicists in Medicine.
[4] Koerkamp, M. L., Vasmel, J. E., Rusell, N. S., Shaitelman, S. F., & Anandadas, C. N. (2020). Optimizing MR-guided radiotherapy for breast cancer patients. Frontiers in Oncology, 10(1107).
[5] Kurz, C., Buizza, G., & Landry, G. (2020). Medical physics challenges in clinical MR-guided radiotherapy. Radiation Oncology, 15(93).
[6] Liney, G. P., Whelan, B., Orbon, B., Barton, M., & Keall, P. (2018). MRI-linear accelerator radiotherapy systems. Clinical Oncology, 30.
[7] Patterson, E., Oborn, B. M., Cutajar, D., Jelen, U., Liney, G., Rosenfeld, A., & Metcalfe, P. E. (2021). Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-linac. Journal of Applied Clinical Medical Physics.
[8] Raaijmakers, A. J., Raaymakers, B. W., & Lagendijk, J. J. (2005). Integrating an MRI scanner with a six mv radiotherapy accelerator: dose increases at tissue-air interfaces in a lateral magnetic field due to returning electrons. Physics in Medicine & Biology, 50(7).
[9] Raaymakers, B. W. Raaijmakers, A. J. Kotte, A. N. Jette, D., & Lagendijk, J. J. (2004). Integrating an MRI scanner with a six mv radiotherapy accelerator: dose deposition in a transverse magnetic field. Phys Med Biol., 49(17).
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