The ViewRay System: Magnetic Resonance– Guided and Controlled Radiotherapy Sasa Mutic, PhD,* and James F. Dempsey, PhD† A description of the first commercially available magnetic resonance imaging (MRI)–guided radiation therapy (RT) system is provided. The system consists of a split 0.35-T MR scanner straddling 3 60Co heads mounted on a ring gantry, each head equipped with independent doubly focused multileaf collimators. The MR and RT systems share a common isocenter, enabling simultaneous and continuous MRI during RT delivery. An on-couch adaptive RT treatment-planning system and integrated MRI-guided RT control system allow for rapid adaptive planning and beam delivery control based on the visualization of soft tissues. Treatment of patients with this system commenced at Washington University in January 2014. Semin Radiat Oncol 24:196-199 C 2014 Elsevier Inc. All rights reserved.
Overview
T
he ViewRay System (VRS) is an integrated magnetic resonance (MR)–guided radiation therapy (RT) instrument designed to provide simultaneous MR imaging (MRI) and a range of external-beam RT options (from very basic fields to complete on-couch adaptive and MR-controlled intensitymodulated RT [IMRT]) at the same isocenter.1-3 The instrument consists of 3 main components: (1) The MRI—a vertically gapped (double donut) horizontal solenoidal superconducting 0.35-T wholebody MRI. The vertical gap in the main magnet coincides with the 50-cm diameter spherical imaging field of view. The self-shielded gradient coils are thrust and torque compensated to minimize loads and noise during use. The split gradient coil has an inner diameter of 80 cm with a thin, cylindrical, inner former connecting the halves across a 20-cm central section where no windings or electrical connections are present. All connections and cooling channels are made from the outer ends of the coil. The VRS has a maximum gradient strength of *Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO. †ViewRay Incorporated, Oakwood Village, OH. Conflict of interest: (1) Sasa Mutic works as a Consultant for ViewRay Incorporated, (2) James F. Dempsey is the Founder, CSO, shareholder, and member of the Board of Directors of ViewRay Incorporated. Address reprint requests to Sasa Mutic, PhD, Department of Radiation Oncology, Washington University School of Medicine, 4921 Parkview Place, St. Louis, MO 63110. E-mail: [email protected]
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18 mT/m and a maximum slew rate of 200 T/m/s on each axis. The whole-body radio frequency (RF) transmit coil is a 75-cm diameter, 16-rung quadrature birdcage coil with capacitance distributed along the end rings and the rungs. The RF shield is integrated with the wholebody radio frequency transmit coil to form a modular coil or shield unit that is designed to span the magnet gap and yet is thin and uniformly attenuating to prevent beam heterogeneities. Surface MRI array coils are designed to be thin and uniformly attenuating, with approximately 0.75% beam attenuation, and are covered with lowdensity foam to prevent increased surface dose and for improved patient comfort. The minimum-required finished room size to install a VRS is 19 ft 2 in 24 ft 8 in (582.9 751.8 cm2), the minimum-required finished ceiling height is 9 ft 6 in (289.6 cm) (including recommended RF shielding), and, because of the superconducting magnet design that can be disassembled, the VRS can be installed through a minimum clear opening (eg, openings of hallways, doors, and mazes) of 3 ft 11 in 6 ft 11 in (119.4 210.8 cm2). Aside from radiation shielding sufficient for 60Co beams, the vault must also be modified to accommodate an 8-in (20.32 cm) stainless steel He vent pipe from the magnet and at least 6 conduits of 8-in diameter. (2) The RT delivery system—a robotic 3-headed 60Co system that provides a dose rate of 550 cGy/min from three 10.5- 10.5-cm2 fields to the isocenter at installation, coupled with fast, pneumatic, sourcestrobing mechanisms. Overall, 3 sources provide a total dose rate comparable to that of conventional linear
The ViewRay system
197
Figure 1 (A) Cut-away view of the ViewRay System exposing the quadrant of the MRI bore through the 28-cm gap and half of a radiation therapy head at 901. (B) A panoramic photograph of the first ViewRay System clinical installation in vault 1 at the Siteman Cancer Center at Washington University School of Medicine in St. Louis, MO. (Color version of figure is available online.)
accelerators as well as simultaneous delivery from 3 gantry angles that are 1201 apart. The VRS design enables axial radiation beam access to the patient with minimal attenuation. A 3-dimensional computer-aided design drawing of the system with a quadrant cut-away view demonstrating the location of the RT gantry isocenter in the center of the MR scanner field of view is shown in Figure 1A clinical installation at Washington University is shown in Figure 1B. During treatment, MR treatment control is performed by tracking targets or critical structures that are observable in continuous and simultaneous fast planar images, in 1 sagittal plane at 4 frames-per-second or in 3 parallel sagittal planes at 2 frames-per-second, via real-time deformable image registration–based beam control, where soft tissues of interest are tracked in planar images and the beams are only enabled when the tracked tissues are determined to be within the prescribed boundary with approximately 300-ms latency. (3) The adaptive RT (ART) treatment-planning system (TPS)—an integrated high-performance computing planning and delivery software capable of autocontouring, a priori Monte Carlo dose computation, and IMRT or conformal RT planning or both are used to support 3-dimensional conformal RT, IMRT, and on-couch ART. The speed of the TPS (9 field plans with complete optimization, leaf motion calculation, and dose calculations can be
accomplished in less than 30 seconds) enables ART treatments based on the volumetric image of the day. Screen captures from the adaptive planning workflow can be seen in Figure 2. The TPS has options of calculating dose distributions with and without effects of the magnetic field.
Advantages and Disadvantages The major disadvantages of the VRS are (1) the characteristic of the 60Co radiation beam and (2) the low, 0.35-T field strength of the MRI. 60Co is known to provide beams with lower output, less penetration, larger penumbra, and higher surface doses than a linear accelerator.4,5 Low-field MRI is known to provide images with less signal-to-noise and shorter relaxation times. The advantage of 60Co beams is that radioactive decay does not interfere with operation of an MRI unit. By modernizing the cobalt unit, the truly double-focused multileaf collimators provide a penumbra that deceases with field size and provides linac equivalent values for small fields of a few centimeters. Therefore, IMRT optimization mitigates the issues with penetration and penumbra. The surface dose is greatly lowered by the MRI magnetic field sweeping away contamination electrons6 and the output is addressed by the 3-headed RT unit. The advantages of low-field MRI are (1) diminished
S. Mutic, J.F. Dempsey
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Figure 2 (A) A screen capture of the daily MR image of a patient with breast cancer treated with APBI from the ViewRay (left —original, right—daily treatment) System, (B) a corresponding daily autocontoured and reoptimized IMRT treatment plan compared with that of the original (left—planned, right—daily treatment) ABPI, accelerated partial-breast irradiation.
magnetic susceptibility artifacts preventing distortions of the patient (o 0.3 mm, Stanescu et al7), (2) a very low specific absorption rate8 preventing patient heating, and (3) minimal perturbations of the dose distributions and surface doses.9,10 The VRS has entered routine clinical use at Washington University in St. Louis (Fig. 1B) as the world's first
commercially available, Food and Drug Administration 510 (k)–cleared,11 MRI-guided adaptive RT system.
References 1. Dempsey JF, Benoit D, Fitzsimmons JR, et al: A device for realtime 3D image-guided IMRT. Int J Radiat Oncol Biol Phys 63(S202):1, 2005 [poster presentation]
The ViewRay system 2. Dempsey J, Dionne B, Fitzsimmons J, et al: A real‐time MRI guided external beam radiotherapy delivery system. Med Phys 33:2254, 2006 [oral presentation] 3. Mutic S: First commercial hybrid MRI‐IMRT system. Med Phys 39:3934, 2012 [oral presentation] 4. Laughlin JS, Mohan R, Kutcher GJ: Choice of optimum megavoltage for accelerators for photon beam treatment. Int J Radiat Oncol Biol Phys 12 (9):1551-1557, 1986 5. Suit HD: What's the optimum choice? Int J Radiat Oncol Biol Phys 12 (9):1711-1712, 1986 6. Jursinic PA, Mackie TR: Characteristics of secondary electrons produced by 6, 10 and 24 MV x-ray beams. Phys Med Biol 41(8):1499-1509, 1996 7. Stanescu T, Wachowicz K, Jaffray DA: Characterization of tissue magnetic susceptibility-induced distortions for MRIgRT. Med Phys 39 (12):7185-7193, http://dx.doi.org/10.1118/1.4764481
199 8. Findlay RP, Dimbylow PJ: FDTD calculations of specific energy absorption rate in a seated voxel model of the human body from 10 MHz to 3 GHz. Phys Med Biol 51(9):2339-2352, 2006 [Epub 2006 Apr 19] 9. Raaymakers BW, Raaijmakers AJ, Kotte AN, Jette D, Lagendijk JJ: Integrating a MRI scanner with a 6 MV radiotherapy accelerator: Dose deposition in a transverse magnetic field. Phys Med Biol 49 (17):4109-4118, 2004 10. Raaijmakers AJ, Raaymakers BW, Lagendijk JJ: Integrating a MRI scanner with a 6 MV radiotherapy accelerator: Dose increase at tissue-air interfaces in a lateral magnetic field due to returning electrons. Phys Med Biol 50 (7):1363-1376, 2005 [Epub 2005 Mar 16] 11. Medical Device: 21CFR892.5750 Radionuclide Radiation Therapy System Premarket notification number: K111862, May 22, 2012, ViewRay System for Radiation Therapy Premarket notification number: K102915, November 10, 2010, ViewRay Treatment Planning and Delivery System
Overview
T
he ViewRay System (VRS) is an integrated magnetic resonance (MR)–guided radiation therapy (RT) instrument designed to provide simultaneous MR imaging (MRI) and a range of external-beam RT options (from very basic fields to complete on-couch adaptive and MR-controlled intensitymodulated RT [IMRT]) at the same isocenter.1-3 The instrument consists of 3 main components: (1) The MRI—a vertically gapped (double donut) horizontal solenoidal superconducting 0.35-T wholebody MRI. The vertical gap in the main magnet coincides with the 50-cm diameter spherical imaging field of view. The self-shielded gradient coils are thrust and torque compensated to minimize loads and noise during use. The split gradient coil has an inner diameter of 80 cm with a thin, cylindrical, inner former connecting the halves across a 20-cm central section where no windings or electrical connections are present. All connections and cooling channels are made from the outer ends of the coil. The VRS has a maximum gradient strength of *Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO. †ViewRay Incorporated, Oakwood Village, OH. Conflict of interest: (1) Sasa Mutic works as a Consultant for ViewRay Incorporated, (2) James F. Dempsey is the Founder, CSO, shareholder, and member of the Board of Directors of ViewRay Incorporated. Address reprint requests to Sasa Mutic, PhD, Department of Radiation Oncology, Washington University School of Medicine, 4921 Parkview Place, St. Louis, MO 63110. E-mail: [email protected]
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http://dx.doi.org/10.1016/j.semradonc.2014.02.008 1053-4296/& 2014 Elsevier Inc. All rights reserved.
18 mT/m and a maximum slew rate of 200 T/m/s on each axis. The whole-body radio frequency (RF) transmit coil is a 75-cm diameter, 16-rung quadrature birdcage coil with capacitance distributed along the end rings and the rungs. The RF shield is integrated with the wholebody radio frequency transmit coil to form a modular coil or shield unit that is designed to span the magnet gap and yet is thin and uniformly attenuating to prevent beam heterogeneities. Surface MRI array coils are designed to be thin and uniformly attenuating, with approximately 0.75% beam attenuation, and are covered with lowdensity foam to prevent increased surface dose and for improved patient comfort. The minimum-required finished room size to install a VRS is 19 ft 2 in 24 ft 8 in (582.9 751.8 cm2), the minimum-required finished ceiling height is 9 ft 6 in (289.6 cm) (including recommended RF shielding), and, because of the superconducting magnet design that can be disassembled, the VRS can be installed through a minimum clear opening (eg, openings of hallways, doors, and mazes) of 3 ft 11 in 6 ft 11 in (119.4 210.8 cm2). Aside from radiation shielding sufficient for 60Co beams, the vault must also be modified to accommodate an 8-in (20.32 cm) stainless steel He vent pipe from the magnet and at least 6 conduits of 8-in diameter. (2) The RT delivery system—a robotic 3-headed 60Co system that provides a dose rate of 550 cGy/min from three 10.5- 10.5-cm2 fields to the isocenter at installation, coupled with fast, pneumatic, sourcestrobing mechanisms. Overall, 3 sources provide a total dose rate comparable to that of conventional linear
The ViewRay system
197
Figure 1 (A) Cut-away view of the ViewRay System exposing the quadrant of the MRI bore through the 28-cm gap and half of a radiation therapy head at 901. (B) A panoramic photograph of the first ViewRay System clinical installation in vault 1 at the Siteman Cancer Center at Washington University School of Medicine in St. Louis, MO. (Color version of figure is available online.)
accelerators as well as simultaneous delivery from 3 gantry angles that are 1201 apart. The VRS design enables axial radiation beam access to the patient with minimal attenuation. A 3-dimensional computer-aided design drawing of the system with a quadrant cut-away view demonstrating the location of the RT gantry isocenter in the center of the MR scanner field of view is shown in Figure 1A clinical installation at Washington University is shown in Figure 1B. During treatment, MR treatment control is performed by tracking targets or critical structures that are observable in continuous and simultaneous fast planar images, in 1 sagittal plane at 4 frames-per-second or in 3 parallel sagittal planes at 2 frames-per-second, via real-time deformable image registration–based beam control, where soft tissues of interest are tracked in planar images and the beams are only enabled when the tracked tissues are determined to be within the prescribed boundary with approximately 300-ms latency. (3) The adaptive RT (ART) treatment-planning system (TPS)—an integrated high-performance computing planning and delivery software capable of autocontouring, a priori Monte Carlo dose computation, and IMRT or conformal RT planning or both are used to support 3-dimensional conformal RT, IMRT, and on-couch ART. The speed of the TPS (9 field plans with complete optimization, leaf motion calculation, and dose calculations can be
accomplished in less than 30 seconds) enables ART treatments based on the volumetric image of the day. Screen captures from the adaptive planning workflow can be seen in Figure 2. The TPS has options of calculating dose distributions with and without effects of the magnetic field.
Advantages and Disadvantages The major disadvantages of the VRS are (1) the characteristic of the 60Co radiation beam and (2) the low, 0.35-T field strength of the MRI. 60Co is known to provide beams with lower output, less penetration, larger penumbra, and higher surface doses than a linear accelerator.4,5 Low-field MRI is known to provide images with less signal-to-noise and shorter relaxation times. The advantage of 60Co beams is that radioactive decay does not interfere with operation of an MRI unit. By modernizing the cobalt unit, the truly double-focused multileaf collimators provide a penumbra that deceases with field size and provides linac equivalent values for small fields of a few centimeters. Therefore, IMRT optimization mitigates the issues with penetration and penumbra. The surface dose is greatly lowered by the MRI magnetic field sweeping away contamination electrons6 and the output is addressed by the 3-headed RT unit. The advantages of low-field MRI are (1) diminished
S. Mutic, J.F. Dempsey
198
Figure 2 (A) A screen capture of the daily MR image of a patient with breast cancer treated with APBI from the ViewRay (left —original, right—daily treatment) System, (B) a corresponding daily autocontoured and reoptimized IMRT treatment plan compared with that of the original (left—planned, right—daily treatment) ABPI, accelerated partial-breast irradiation.
magnetic susceptibility artifacts preventing distortions of the patient (o 0.3 mm, Stanescu et al7), (2) a very low specific absorption rate8 preventing patient heating, and (3) minimal perturbations of the dose distributions and surface doses.9,10 The VRS has entered routine clinical use at Washington University in St. Louis (Fig. 1B) as the world's first
commercially available, Food and Drug Administration 510 (k)–cleared,11 MRI-guided adaptive RT system.
References 1. Dempsey JF, Benoit D, Fitzsimmons JR, et al: A device for realtime 3D image-guided IMRT. Int J Radiat Oncol Biol Phys 63(S202):1, 2005 [poster presentation]
The ViewRay system 2. Dempsey J, Dionne B, Fitzsimmons J, et al: A real‐time MRI guided external beam radiotherapy delivery system. Med Phys 33:2254, 2006 [oral presentation] 3. Mutic S: First commercial hybrid MRI‐IMRT system. Med Phys 39:3934, 2012 [oral presentation] 4. Laughlin JS, Mohan R, Kutcher GJ: Choice of optimum megavoltage for accelerators for photon beam treatment. Int J Radiat Oncol Biol Phys 12 (9):1551-1557, 1986 5. Suit HD: What's the optimum choice? Int J Radiat Oncol Biol Phys 12 (9):1711-1712, 1986 6. Jursinic PA, Mackie TR: Characteristics of secondary electrons produced by 6, 10 and 24 MV x-ray beams. Phys Med Biol 41(8):1499-1509, 1996 7. Stanescu T, Wachowicz K, Jaffray DA: Characterization of tissue magnetic susceptibility-induced distortions for MRIgRT. Med Phys 39 (12):7185-7193, http://dx.doi.org/10.1118/1.4764481
199 8. Findlay RP, Dimbylow PJ: FDTD calculations of specific energy absorption rate in a seated voxel model of the human body from 10 MHz to 3 GHz. Phys Med Biol 51(9):2339-2352, 2006 [Epub 2006 Apr 19] 9. Raaymakers BW, Raaijmakers AJ, Kotte AN, Jette D, Lagendijk JJ: Integrating a MRI scanner with a 6 MV radiotherapy accelerator: Dose deposition in a transverse magnetic field. Phys Med Biol 49 (17):4109-4118, 2004 10. Raaijmakers AJ, Raaymakers BW, Lagendijk JJ: Integrating a MRI scanner with a 6 MV radiotherapy accelerator: Dose increase at tissue-air interfaces in a lateral magnetic field due to returning electrons. Phys Med Biol 50 (7):1363-1376, 2005 [Epub 2005 Mar 16] 11. Medical Device: 21CFR892.5750 Radionuclide Radiation Therapy System Premarket notification number: K111862, May 22, 2012, ViewRay System for Radiation Therapy Premarket notification number: K102915, November 10, 2010, ViewRay Treatment Planning and Delivery System
