Monte Carlo simulations of out-of-field skin dose due to spiralling contaminant electrons in a perpendicular magnetic field

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

PURPOSE

To 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.

METHOD

Skin 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%).

RESULTS

OSLD, 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.

CONCLUSIONS

These 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.

A comparison of measured and treatment planning system out-of-field dose for a 1.5 T MR linac

Objective.Dose due to the electron streaming effect (ESE) is a significant contribution to out-of-field dose on the Elekta Unity MR-Linac. The aim of this work is to provide a systematic comparison of calculated and measured streaming dose for this system.Approach.Beams 1.0 × 1.0 cm2to 5.0 × 5.0 cm2, gantry 90.0°, 1000 MU, were incident on an in-house phantom. At the beam entrance and exit surfaces of the phantom, ESE was generated in theY-direction (IEC 61217). EBT3 film, orientated within theX-Zplane and at 14.0 mm depth in a solid water block, was used to determine ESE dose 5.0 cm beyond the phantom. The experimental arrangement was simulated in the Monaco v5.4 treatment planning system (TPS), utilising a CT phantom dataset with differing relative electron densities (RED) for the surrounding air. Horizontal (Xdirection) and vertical (Zdirection) film dose profiles were compared to the corresponding TPS profiles.Main results. For each field, the maximum ESE dose was observed at the beam exit, the magnitude of which decreases with decreasing field size. For the 5.0 × 5.0 cm2field, the exit and entry ESE doses were 19.6% and 7.0% of theDmaxdose to water, respectively. Across horizontal profiles, differences (simulated-measured) were reduced with smaller fields and lower RED. The maximum absolute profile difference was 1.7% of theDmaxdose to water for optimal RED and isocentre location. In vertical profiles an offset consistent with the Lorentz force was observed relative to theX-Yisoplane.Significance. For the fields investigated, maximum absolute differences (simulated-measured) ≤ 5.2% occurred in peak regions of ESE, at the beam entrance and exit from the phantom. Generally, there is good agreement between Monaco simulated and measured ESE. Simulated out-of-field dose is sensitive to the RED assigned to air structures and unforced RED optimises out-of-field dose calculation accuracy.

Electron stream effect in 0.35 Tesla magnetic resonance image guided radiotherapy for breast cancer

Purpose

This research aimed to analyze electron stream effect (ESE) during magnetic resonance image guided radiotherapy (MRgRT) for breast cancer patients on a MR-Linac (0.35 Tesla, 6MV), with a focus on the prevention of redundant radiation exposure.

Materials and methods

RANDO phantom was used with and without the breast attachment in order to represent the patients after breast conserving surgery (BCS) and those received modified radical mastectomy (MRM). The prescription dose is 40.05 Gy in fifteen fractions for whole breast irradiation (WBI) or 20 Gy single shot for partial breast irradiation (PBI). Thirteen different portals of intensity-modulated radiation therapy were created. And then we evaluated dose distribution in five areas (on the skin of the tip of the nose, the chin, the neck, the abdomen and the thyroid.) outside of the irradiated field with and without 0.35 Tesla. In addition, we added a piece of bolus with the thickness of 1cm on the skin in order to compare the ESE difference with and without a bolus. Lastly, we loaded two patients' images for PBI comparison.

Results

We found that 0.35 Tesla caused redundant doses to the skin of the chin and the neck as high as 9.79% and 5.59% of the prescription dose in the BCS RANDO model, respectively. For RANDO phantom without the breast accessory (simulating MRM), the maximal dose increase were 8.71% and 4.67% of the prescription dose to the skin of the chin and the neck, respectively. Furthermore, the bolus we added efficiently decrease the unnecessary dose caused by ESE up to 59.8%.

Conclusion

We report the first physical investigation on successful avoidance of superfluous doses on a 0.35T MR-Linac for breast cancer patients. Future studies of MRgRT on the individual body shape and its association with ESE influence is warranted.

ACPSEM position paper: the safety of magnetic resonance imaging linear accelerators

Magnetic Resonance Imaging linear-accelerator (MRI-linac) equipment has recently been introduced to multiple centres in Australia and New Zealand. MRI equipment creates hazards for staff, patients and others in the MR environment; these hazards must be well understood, and risks managed by a system of environmental controls, written procedures and a trained workforce. While MRI-linac hazards are similar to the diagnostic paradigm, the equipment, workforce and environment are sufficiently different that additional safety guidance is warranted. In 2019 the Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) formed the Magnetic Resonance Imaging Linear-Accelerator Working Group (MRILWG) to support the safe clinical introduction and optimal use of MR-guided radiation therapy treatment units. This Position Paper is intended to provide safety guidance and education for Medical Physicists and others planning for and working with MRI-linac technology. This document summarises MRI-linac hazards and describes particular effects which arise from the combination of strong magnetic fields with an external radiation treatment beam. This document also provides guidance on safety governance and training, and recommends a system of hazard management tailored to the MRI-linac environment, ancillary equipment, and workforce.

Adaptive radiotherapy for breast cancer

Research in the field of local and locoregional breast cancer radiotherapy aims to maintain excellent oncological outcomes while reducing treatment-related toxicity. Adaptive radiotherapy (ART) considers variations in target and organs at risk (OARs) anatomy occurring during the treatment course and integrates these in re-optimized treatment plans. Exploiting ART routinely in clinic may result in smaller target volumes and better OAR sparing, which may lead to reduction of acute as well as late toxicities. In this review MR-guided and CT-guided ART for breast cancer patients according to different clinical scenarios (neoadjuvant and adjuvant partial breast irradiation, whole breast, chest wall and regional nodal irradiation) are reviewed and their advantages as well as challenging aspects discussed.

Estimation of secondary cancer projected risk after partial breast irradiation at the 1.5 T MR-linac

PURPOSE

For patients treated with partial breast irradiation (PBI), potential long-term treatment-related toxicities are important. The 1.5 T magnetic resonance guided linear accelerator (MRL) offers excellent tumor bed visualization and a daily treatment plan adaption possibility, but MRL-specific electron stream and return effects may cause increased dose deposition at air-tissue interfaces. In this study, we aimed to investigate the projected risk of radiation-induced secondary malignancies (RISM) in patients treated with PBI at the 1.5 T MRL.

METHODS

Projected excess absolute risk values (EARs) for the contralateral breast, lungs, thyroid and esophagus were estimated for 11 patients treated with PBI at the MRL and compared to 11 patients treated with PBI and 11 patients treated with whole breast irradiation (WBI) at the conventional linac (CTL). All patients received 40.05 Gy in 15 fractions. For patients treated at the CTL, additional dose due to daily cone beam computed tomography (CBCT) was simulated. The t‑test with Bonferroni correction was used for comparison.

RESULTS

The highest projected risk for a radiation-induced secondary cancer was found for the ipsilateral lung, without significant differences between the groups. A lower contralateral breast EAR was found for MRL-PBI (EAR = 0.89) compared to CTL-PBI (EAR = 1.41, p = 0.01), whereas a lower thyroid EAR for CTL-PBI (EAR = 0.17) compared to MRL-PBI (EAR = 0.33, p = 0.03) and CTL-WBI (EAR = 0.46, p = 0.002) was observed. Nevertheless, when adding the CBCT dose no difference between thyroid EAR for CTL-PBI compared to MRL-PBI was detected.

CONCLUSION

Better breast tissue visualization and the possibility for daily plan adaption make PBI at the 1.5 T MRL particularly attractive. Our simulations suggest that this treatment can be performed without additional projected risk of RISM.

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

With the clinical introduction of MR-linacs, out-of-field dose (OFD) associated with head leakage/scatter (HLS), spiralling contaminant electrons (SCE) and the electron streaming effect (ESE) is of interest. To investigate HLS and SCE, EBT3 film on solid water 5.0 cm beyond each edge of a 10.0 × 10.0 cm² field was used to determine depth-dose for 0 T and 1.5 T, in the isocentric plane. Additionally, ESE induced by the anterior imaging coil was quantified and the experimental arrangements to measure SCE and ESE were modelled using Monaco. For a clinical treatment of supraclavicular nodal disease, Monaco OFD was compared to in vivo measurements. For 0 T, depth-dose was isotropic and surface dose was approximately 4.4% of D_max. With 1.5 T surface doses were approximately 3.8% of D_max at ± Y (IEC61217), compared to 2.6% and 0.6% of D_max at - X and X, respectively. For both field strengths, the TPS depth-dose variation was consistent with experimental trends; however, near surface doses calculated at ± Y differed significantly from measurements. For the field sizes investigated, measured coil ESE dose was between 9.0 and 28.0% of D_max and Monaco coil ESE was less than measured by up to 13.0%. OFD in 0 T and 1.5 T are comparable at ± Y, inconsistent with previous work. Anterior coil ESE should be mitigated during treatment and for the clinical case investigated, in vivo OFD was within 2σ of TPS calculations. Monaco overestimates near surface SCE and underestimates coil ESE.

Reference dosimetry in MRI-linacs: evaluation of available protocols and data to establish a Code of Practice

With the rapid increase in clinical treatments with MRI-linacs, a consistent, harmonized and sustainable ground for reference dosimetry in MRI-linacs is needed. Specific for reference dosimetry in MRI-linacs is the presence of a strong magnetic field. Therefore, existing Code of Practices (CoPs) are inadequate. In recent years, a vast amount of papers have been published in relation to this topic. The purpose of this review paper is twofold: to give an overview and evaluate the existing literature for reference dosimetry in MRI-linacs and to discuss whether the literature and datasets are adequate and complete to serve as a basis for the development of a new or to extend existing CoPs. This review is prefaced with an overview of existing MRI-linac facilities. Then an introduction on the physics of radiation transport in magnetic fields is given. The main part of the review is devoted to the evaluation of the literature with respect to the following subjects: • beam characteristics of MRI-linac facilities; • formalisms for reference dosimetry in MRI-linacs; • characteristics of ionization chambers in the presence of magnetic fields; • ionization chamber beam quality correction factors; and • ionization chamber magnetic field correction factors. The review is completed with a discussion as to whether the existing literature is adequate to serve as basis for a CoP. In addition, it highlights subjects for future research on this topic.

Analysis of the electron-stream effect in patients treated with partial breast irradiation using the 1.5 T MR-linear accelerator

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The electron-stream effect is responsible of an out-of-field dose when performing partial breast irradiation at the MR-linac.

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The electron-stream effect is located to the patient́s chin and, for lateral targets, to the arm.

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The electron-stream effect is effectively minimized by a bolus.

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Novel concepts and clinical trials for breast cancer at the 1.5 T MR-linac can be tested.

Introduction

The hybrid magnetic resonance linear accelerator (MRL) has the potential to test novel concepts in breast cancer patients such as daily MR-guided real-time plan adaptation. Before starting clinical trials, preparatory studies for example of the MR-dependent electron stream effect (ESE) are necessary.

Material and Methods

To prospectively investigate the ESE, data from 11 patients treated with partial breast irradiation (PBI) at the 1.5 T MRL were evaluated. A bolus was placed on the chin and in vivo dosimetry results were compared with the dose simulated by the treatment planning system (TPS). The same measurements were carried out for three patients treated at a conventional linac. Toxicity and cosmesis were evaluated.

Results

Median doses measured and simulated on top/ underneath the bolus were 1.91 / 0.62 Gy and 2.82 / 0.63 Gy, respectively. Median differences between calculations and measurements were 0.8 Gy and 0.1 Gy. At the conventional linac, median measured doses on top/ underneath the bolus were 0.98 and 1.37 Gy. No acute toxicity exceeding grade 2 was recorded. Cosmesis was good or excellent and patient reported outcome measures were mostly scored as none or mild.

Conclusion

The dose due to the ESE is low, correctly predicted by the TPS and effectively minimized by a bolus.

In-Air Electron Streaming Effect for Esophageal Cancer Radiotherapy With a 1.5 T Perpendicular Magnetic Field: A Treatment Planning Study

Purpose

To investigate the in-air out-of-field electron streaming effect (ESE) for esophageal cancer radiotherapy in the presence of 1.5 T perpendicular magnetic field.

Methods

Ten esophageal cancer patients treated with conventional Linac were retrospectively enrolled into a cohort of this study, with the prescription of 4,400 cGy/20 fx. All cases received IMRT replanning using Elekta Unity MR-Linac specified Monaco system, denoted as primary plan. To visualize the in-air dose outside the body in Monaco system, an auxiliary structure was created by extending the external structure. For each case, another comparable plan with no magnetic field was created using the same planning parameters. The plan was also recalculated by placing a bolus upon the neck and chin area to investigate its shielding effect for ESE. Dosimetric evaluations of the out-of-field neck and chin skin area and statistical analysis for these plans were then performed.

Results

Out-of-field ESE was also observed in esophageal cancer treatment planning under 1.5 T magnetic field, while totally absent for plans with no magnetic field. On average, the maximum dose to the neck and chin skin area of the primary plan (657.92 ± 69.07 cGy) was higher than that of plan with no magnetic field (281.78 ± 36.59 cGy, p = 0.005) and plan with bolus (398.43 ± 69.19 cGy, p = 0.007). DVH metrics D_1cc (the minimum dose to 1 cc volume) of the neck and chin skin for primary plan was 382.06 ± 44.14 cGy, which can be reduced to 212.42 ± 23.65 cGy by using the 1 cm bolus (with p = 0.005), even lower than the plan without magnetic field (214.45 ± 23.82, p = 0.005). No statistically significant difference of the neck and chin skin dose between the plan with bolus and plan with no magnetic field was observed (all with p > 0.05).

Conclusion

For MRI guided esophageal cancer radiotherapy, a relatively high out-of-field neck and chin skin doses will be introduced by ESE in the presence of magnetic field. It is therefore recommended to take this into account during the planning phase. Adding bolus could effectively reduce the ESE dose contributions, achieve the shielding effect almost equivalent to the scenario with no magnetic field. Further explorations of measurement verifications for the ESE dose distributions are required.