Sources of out-of-field dose in MRgRT: an inter-comparison of measured and Monaco treatment planning system doses for the Elekta Unity MR-linac

Baines, John; Powers, Marcus; Newman, Glen

doi:10.1007/s13246-021-01039-6

Cited by 8 publications

(40 citation statements)

References 16 publications

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“…Measured and calculated attenuation were consistent to within 0.5%, with the largest discrepancy occurring at G30. Whilst the attenuation of the beam due to the anterior coil is small, the effect this has on out-of-field doses is not [ 32 ]. During patient treatments on the Unity system, where multiple gantry angles would be used, the effect of the anterior coil attenuation on the delivered dose would be negligible; however, the potential ESE should be investigated, and appropriate patient shielding should be provided [ 32 ].…”

Section: Discussionmentioning

confidence: 99%

“…Whilst the attenuation of the beam due to the anterior coil is small, the effect this has on out-of-field doses is not [ 32 ]. During patient treatments on the Unity system, where multiple gantry angles would be used, the effect of the anterior coil attenuation on the delivered dose would be negligible; however, the potential ESE should be investigated, and appropriate patient shielding should be provided [ 32 ]. Note that angular dependent output through the other attenuating objects (couch and cryostat) for this system have been published elsewhere [ 33 ].…”

Section: Discussionmentioning

confidence: 99%

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Commissioning measurements on an Elekta Unity MR-Linac

et al. 2022

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Magnetic resonance-guided radiotherapy technology is relatively new and commissioning publications, quality assurance (QA) protocols and commercial products are limited. This work provides guidance for implementation measurements that may be performed on the Elekta Unity MR-Linac (Elekta, Stockholm, Sweden). Adaptations of vendor supplied phantoms facilitated determination of gantry angle accuracy and linac isocentre, whereas in-house developed phantoms were used for end-to-end testing and anterior coil attenuation measurements. Third-party devices were used for measuring beam quality, reference dosimetry and during treatment plan commissioning; however, due to several challenges, variations on standard techniques were required. Gantry angle accuracy was within 0.1°, confirmed with pixel intensity profiles, and MV isocentre diameter was < 0.5 mm. Anterior coil attenuation was approximately 0.6%. Beam quality as determined by TPR20,10 was 0.705 ± 0.001, in agreement with treatment planning system (TPS) calculations, and gamma comparison against the TPS for a 22.0 × 22.0 cm2 field was above 95.0% (2.0%, 2.0 mm). Machine output was 1.000 ± 0.002 Gy per 100 MU, depth 5.0 cm. During treatment plan commissioning, sub-standard results indicated issues with machine behaviour. Once rectified, gamma comparisons were above 95.0% (2.0%, 2.0 mm). Centres which may not have access to specialized equipment can use in-house developed phantoms, or adapt those supplied by the vendor, to perform commissioning work and confirm operation of the MRL within published tolerances. The plan QA techniques used in this work can highlight issues with machine behaviour when appropriate gamma criteria are set.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Commissioning measurements on an Elekta Unity MR-Linac

et al. 2022

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“…The magnitude of coil ESE dose and the comparable readings at either end of the coil will require shielding at both locations to protect patients. Of course, if the coil is horizontal, the ESE dose would be minimized (Figure 4F and Figure 8E) [14]. This may not be possible in all scenarios, particularly when the patient geometry does not allow for it and MR image quality would be significantly impacted.…”

Section: Clinical Casementioning

confidence: 99%

“…In addition, Lorentz interactions cause contaminant electrons in the primary X-ray beam to spiral along the direction of B 0 . Spiraling contaminant electrons (SCE) also contribute to OFD in the cranio-caudal direction [13,14].…”

Section: Introductionmentioning

confidence: 99%

“…To facilitate imaging of the tumor site, an MR receiver coil is positioned anteriorly above the requisite patient anatomy. A previous work has suggested that irradiation of the tilted anterior coil induces clinically significant OFD [14]; however, a thorough characterization of this phenomenon is lacking. In this work, we report on ESE originating from irradiation of this anterior imaging coil.…”

Section: Introductionmentioning

confidence: 99%

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Electron Streaming Effect Associated With the Elekta Unity Anterior Imaging Coil

Powers

Baines

2022

Front. Phys.

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The presence of the static field in magnetic resonance guided radiation therapy (MRgRT) systems, such as the Elekta Unity MR-Linac (MRL), influences charged particle motion due to the Lorentz force in and around patients. Consequently, for the Unity, changes to out-of-field dose (OFD) relative to conventional linacs occur due to the electron streaming effect (ESE) and spiraling contaminant electrons (SCE). This work investigates OFD associated with irradiation of the anterior MR imaging coil, which is in situ for all treatments on the Unity. Film measurements and Monaco simulations were performed to quantify the magnitude of OFD at the superior and inferior ends of the coil as a function of coil tilt relative to the beam direction. The dependence of OFD on field size and the relative electron density (RED) assigned to the coil and surrounding air are reported. The doses at both coil ends were clinically significant, with nearly 23.0% of the Dmax dose to water being recorded for the largest field (8.0 × 22.0 cm2) and 6.8% for the smallest field (3.0 × 3.0 cm2). Monaco simulations of OFD agreed with film within 5.0%, when appropriate calculation conditions were set. OFD decreases as coil tilt is reduced, and there is no evidence of ESE when the coil is horizontal. Clinically, the potential magnitude of cranio–caudal streaming dose from a tilted coil necessitates the use of appropriate shielding. A clinical case involving coil-induced ESE during treatment of a lesion in the right angle is presented. The planning-based investigation revealed that ESE doses associated with the coil and an immobilization vacuum bag can be clinically significant.

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Electron streaming dose measurements and calculations on a 1.5 T MR‐Linac

Patterson,

Powers,

Metcalfe

et al. 2024

J Applied Clin Med Phys

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PurposeTo evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)‐linac.MethodSkin dose due to the ESE on an MR‐linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x‐y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22 cm2 fields, gantry at 90°. Out‐of‐field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed ±20.0 cm from the isocenter along the x‐direction. A high‐resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07 mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1 cm dose grid with 0.2% statistical uncertainty (0.1 cm, 0.2%) and a 0.2 cm dose grid with 3.0% statistical uncertainty (0.2 cm, 3.0%).ResultsOSLD, film, the 0.1 cm, 0.2%, and 0.2 cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as –75.3%, –7.0%, –24.7%, and –41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements.ConclusionsThese results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

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Sources of out-of-field dose in MRgRT: an inter-comparison of measured and Monaco treatment planning system doses for the Elekta Unity MR-linac

Cited by 8 publications

References 16 publications

Commissioning measurements on an Elekta Unity MR-Linac

Commissioning measurements on an Elekta Unity MR-Linac

Electron Streaming Effect Associated With the Elekta Unity Anterior Imaging Coil

Electron streaming dose measurements and calculations on a 1.5 T MR‐Linac

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