Lung stereotactic body radiotherapy with an MR-linac – Quantifying the impact of the magnetic field and real-time tumor tracking

Menten, Martin J.; Fast, Martin F.; Nill, Simeon; Kamerling, C P; McDonald, Fiona; Oelfke, Uwe

doi:10.1016/j.radonc.2016.04.019

Cited by 94 publications

(106 citation statements)

References 31 publications

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“…For our cohort, the average MLD presented was estimated to be above this 4 Gy cut-off value with ITV-based planning, and was lowered below 4 Gy with MLC tracking (3.5 Gy). A recent study simulated the effect of MLC tracking during MR-LINAC treatment and showed a mean lung dose reduction of 0.3 Gy, against our study with 0.8 Gy [33].…”

Section: Dose Reconstruction Limitationssupporting

confidence: 39%

MLC tracking for lung SABR reduces planning target volumes and dose to organs at risk

Caillet

Keall

Colvill

et al. 2017

Radiotherapy and Oncology

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Section: Dose Reconstruction Limitationssupporting

confidence: 39%

MLC tracking for lung SABR reduces planning target volumes and dose to organs at risk

Caillet

Keall

Colvill

et al. 2017

Radiotherapy and Oncology

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“…The impact of the magnetic field on electron trajectories, and consequently on dose distributions,, has been well established. A magnetic field orthogonal to the incoming radiation beam induces several effects on the dose distribution , , . One of these is the electron return effect (ERE), which occurs in regions of sharp density changes and is responsible for an increase in the interface surface dose.…”

Section: Introductionmentioning

confidence: 99%

“…A magnetic field orthogonal to the incoming radiation beam induces several effects on the dose distribution. 11,12,13 One of these is the electron return effect (ERE), which occurs in regions of sharp density changes and is responsible for an increase in the interface surface dose. Further effects include a dose shift, toward the direction perpendicular to both the beam and magnetic field, and an increase in the maximum dose deposition, due to the electron energy being deposited closer to the point of photon interaction.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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PurposeThe purpose of this study was to evaluate the potential skin dose toxicity contribution of spiralling contaminant electrons (SCE) generated in the air in an MR‐linac with a 0.35 or 1.5 T magnetic field using the EGSnrc Monte Carlo (MC) code. Comparisons to experimental results at 1.5 T are also performed.MethodsAn Elekta generated phase space file for the Unity MR‐linac is used in conjunction with the EGSnrc enhanced electric and magnetic field transport macros to simulate surface dose profiles and depth‐dose curves in panels located 5 cm away from the beam edge and positioned either parallel or perpendicular to the magnetic field. Electrons generated in the air will spiral along the magnetic field lines, and though surface doses within the field will be reduced, the electrons can contribute to out‐of‐field surface doses.ResultsSurface dose profiles showed good agreement with experimental findings and the maximum simulated doses at surfaces perpendicular to the magnetic field were 3.77 ± 0.01% and 3.55 ± 0.01% for 1.5 and 0.35 T. These results are expressed as a percentage of the maximum dose to water delivered by the photon beam. The surface dose variations in the out‐of‐field region converge to the 0 T doses within the first 0.5 cm of material. An asymmetry in the dose distribution in surfaces positioned on either side of the photon beam and aligned parallel to the magnetic field is determined to be due to the magnetic field directing electrons deeper into, or localizing them to the surface of, the measurement panel.ConclusionsThese results confirm the SCE dose contribution in surfaces perpendicular to the magnetic field and show these doses to be of the order of a few percentage of the maximum dose to water of the beam. Good agreement in the dose profiles is seen in comparisons between the MC simulations and experimental work. The effect is apparent in 0.35 and 1.5 T magnetic fields and dissipates within the first few millimeters of material. It should be noted that only SCEs from beam anteriorly incident on the patient will influence the patient surface dose, and the use of beams incident over different angles will reduce the dose to any particular patient surface.

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“…Assuming that centroid MLC tracking compensates for rigid target shifts, we have created treatment plans for a tracking scenario using the moving target volume (MTV) approach, which was utilized in Menten et al 30 Figure 4 schematically shows how construction of the MTV can be compared to the construction of the conventional ITV. Both methods rely on the GTV being delineated on each individual 4DCT phase.…”

Section: Methodsmentioning

confidence: 99%

Real‐time 4D dose reconstruction for tracked dynamic MLC deliveries for lung SBRT

et al. 2016

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PurposeThis study provides a proof of concept for real‐time 4D dose reconstruction for lung stereotactic body radiation therapy (SBRT) with multileaf collimator (MLC) tracking and assesses the impact of tumor tracking on the size of target margins.MethodsThe authors have implemented real‐time 4D dose reconstruction by connecting their tracking and delivery software to an Agility MLC at an Elekta Synergy linac and to their in‐house treatment planning software (TPS). Actual MLC apertures and (simulated) target positions are reported to the TPS every 40 ms. The dose is calculated in real‐time from 4DCT data directly after each reported aperture by utilization of precalculated dose‐influence data based on a Monte Carlo algorithm. The dose is accumulated onto the peak‐exhale (reference) phase using energy‐mass transfer mapping. To investigate the impact of a potentially reducible safety margin, the authors have created and delivered treatment plans designed for a conventional internal target volume (ITV) + 5 mm, a midventilation approach, and three tracking scenarios for four lung SBRT patients. For the tracking plans, a moving target volume (MTV) was established by delineating the gross target volume (GTV) on every 4DCT phase. These were rigidly aligned to the reference phase, resulting in a unified maximum GTV to which a 1, 3, or 5 mm isotropic margin was added. All scenarios were planned for 9‐beam step‐and‐shoot IMRT to meet the criteria of RTOG 1021 (3 × 18 Gy). The GTV 3D center‐of‐volume shift varied from 6 to 14 mm.ResultsReal‐time dose reconstruction at 25 Hz could be realized on a single workstation due to the highly efficient implementation of dose calculation and dose accumulation. Decreased PTV margins resulted in inadequate target coverage during untracked deliveries for patients with substantial tumor motion. MLC tracking could ensure the GTV target dose for these patients. Organ‐at‐risk (OAR) doses were consistently reduced by decreased PTV margins. The tracked MTV + 1 mm deliveries resulted in the following OAR dose reductions: lung V 20 up to 3.5%, spinal cord D 2 up to 0.9 Gy/Fx, and proximal airways D 2 up to 1.4 Gy/Fx.ConclusionsThe authors could show that for patient data at clinical resolution and realistic motion conditions, the delivered dose could be reconstructed in 4D for the whole lung volume in real‐time. The dose distributions show that reduced margins yield lower doses to healthy tissue, whilst target dose can be maintained using dynamic MLC tracking.

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Lung stereotactic body radiotherapy with an MR-linac – Quantifying the impact of the magnetic field and real-time tumor tracking

Cited by 94 publications

References 31 publications

MLC tracking for lung SABR reduces planning target volumes and dose to organs at risk

MLC tracking for lung SABR reduces planning target volumes and dose to organs at risk

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

Real‐time 4D dose reconstruction for tracked dynamic MLC deliveries for lung SBRT

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