Dosimetry in the presence of strong magnetic fields

O'Brien, Daniel; Schupp, Nicholas; Pencea, S; Dolan, James P.; Sawakuchi, Gabriel O.

doi:10.1088/1742-6596/847/1/012055

Cited by 17 publications

(15 citation statements)

References 21 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The dose deposited at tissue‐air interfaces can increase due to the electron returning effect (ERE), which can also be affected by the surface orientation where the photon beam enters or exits . The dosimetric response of QA devices (especially the detectors used for beam calibration) in magnetic fields have also been studied, focusing mainly on the absolute dose response of the detectors. The effect of the magnetic field on the detectors is demonstrated not only through the trajectory path deflection of the secondary electrons in the medium surrounding and inside the volume of the gas‐filled or solid detector, but also through the change of the intrinsic properties of detectors such as charge carrier versus lattice defect recombination, polarity effect, etc.…”

Section: Introductionmentioning

confidence: 99%

Measurement validation of treatment planning for a MR‐Linac

Chen

Paulson

Ahunbay

et al. 2019

J Applied Clin Med Phys

View full text Add to dashboard Cite

Purpose The magnetic field can cause a nonnegligible dosimetric effect in an MR‐Linac system. This effect should be accurately accounted for by the beam models in treatment planning systems (TPS). The purpose of the study was to verify the beam model and the entire treatment planning and delivery process for a 1.5 T MR‐Linac based on comprehensive dosimetric measurements and end‐to‐end tests. Material and methods Dosimetry measurements and end‐to‐end tests were performed on a preclinical MR‐Linac (Elekta AB) using a multitude of detectors and were compared to the corresponding beam model calculations from the TPS for the MR‐Linac. Measurement devices included ion chambers (IC), diamond detector, radiochromic film, and MR‐compatible ion chamber array and diode array. The dose in inhomogeneous phantom was also verified. The end‐to‐end tests include the generation, delivery, and comparison of 3D and IMRT plan with measurement. Results For the depth dose measurements with Farmer IC, micro IC and diamond detector, the absolute difference between most measurement points and beam model calculation beyond the buildup region were <1%, at most 2% for a few measurement points. For the beam profile measurements, the absolute differences were no more than 1% outside the penumbra region and no more than 2.5% inside the penumbra region. Results of end‐to‐end tests demonstrated that three 3D static plans with single 5 × 10 cm2 fields (at gantry angle 0°, 90° and 270°) and two IMRT plans successfully passed gamma analysis with clinical criteria. The dose difference in the inhomogeneous phantom between the calculation and measurement was within 1.0%. Conclusions Both relative and absolute dosimetry measurements agreed well with the TPS calculation, indicating that the beam model for MR‐Linac properly accounts for the magnetic field effect. The end‐to‐end tests verified the entire treatment planning process.

show abstract

Section: Introductionmentioning

confidence: 99%

Measurement validation of treatment planning for a MR‐Linac

Chen

Paulson

Ahunbay

et al. 2019

J Applied Clin Med Phys

View full text Add to dashboard Cite

show abstract

“…The strong magnetic field used for imaging in an MRI scanner has a significant effect on the secondary electron trajectories due to Lorentz force, affecting dose distribution in water and the dose response of ionization chambers . This phenomenon has been studied earlier by Smit et al and O'Brien et al Smit et al measured relative response of NE2571 ionization chamber with a geometry that covered a 180° rotation from orientation with chamber's central axis parallel to the magnetic field. O'Brien et al have carried out similar studies for PTW 30013, PTW 30010, and NE2571 chambers.…”

Section: Introductionmentioning

confidence: 99%

Experimental measurement of ionization chamber angular response and associated magnetic field correction factors in MR‐linac

et al. 2020

View full text Add to dashboard Cite

Purpose: To measure ionization chamber dose response as a function of the angle between magnetic field direction and ionization chamber orientation in magnetic resonance-guided radiation therapy (MRgRT) system, and to evaluate angular dependence of magnetic field correction factor for reference dosimetry. Methods: Measurements were performed on an Elekta MR-linac that integrates a 1.5-T Philips MRI and a 7-MV FFF photon beam accelerator. The response of four reference class chambers (Exradin-A19, A1SL, IBA FC65-G, and CC13, paired with a PTW UE electrometer) was studied. An in-house built MR-compatible water tank and an accompanying cylindrical insert that allowed chamber rotation around the cylinder's axis was used. The EPID onboard imaging was used to center chamber at the MR-linac isocenter (143.5 cm, SAD), as well as to verify position at each datapoint. Results: A clear angular dependence of dose response for all chambers has been measured. The most significant effect of magnetic field on relative chamber response in the presence of magnetic field was observed in the orientation when chamber axis is perpendicular to the direction of magnetic field with the tip pointing in the same direction as Lorentz force. This effect is more pronounced for larger volume chambers; the maximum relative variation in the chamber response (between the setup described above and the one where chamber and magnetic field are parallel) is a 5.3% and 4.6% increase for A19 and FC65-G, respectively, and only 2.0% and 1.9% for smaller volume A1SL and CC13 chamber, respectively. We measured the absolute magnitude of the magnetic field correction factor k mag Q for the Exradin-A19, A1SL, IBA FC65-G, and CC13 to be 0.938 AE 1.13%, 0.968 AE 0.99%, 0.950 AE 1.13%, and 0.975 AE 1.13%, respectively. The values are for perpendicular orientation of the chamber relative to magnetic field and parallel to the Lorentz force. Conclusions: Experimental measurements carried out in this study have verified the optimal orientation of ionization chamber in terms of minimizing effect of magnetic field on the chamber dose response. This study provides a detailed high-resolution measurement of absolute k mag Q values for four reference class chambers as a function of the angle between ionization chamber's central axis and the direction of strong magnetic field over a full 360°rotation. The experimental results of this study can further be used for optimization of the actual sensitive volume of the chamber (and analysis of dead volume) in future Monte Carlo chamber simulations in the presence of strong magnetic fields. In addition, it will provide some necessary data for future reference dosimetry protocols for MR-linac.

show abstract

“…68 Chamber response is most stable and the k Q, B correction value for chambers is less than 1% when the chamber's long axis is aligned parallel to the magnetic field, compared with ∼5% correction if aligned perpendicular to the magnetic field. 58,70,78,79 Water equivalent phantom should be used with caution as submillimeter air gaps between a chamber and the phantom insert can significantly affect the response…”

Section: Absolute Dosimetry In the Mr-linacsmentioning

confidence: 99%

Magnetic field induced dose effects in radiation therapy using MR‐linacs

et al. 2023

View full text Add to dashboard Cite

MR-guided radiotherapy (MRgRT) is one of the most significant advances in radiotherapy in recent years. The hybrid systems were designed to visualize patient anatomical and physiological changes during the course of radiotherapy, enabling more precise treatment. However, before MR-linacs reach their full potential in delivering safe and accurate treatments to patients, the radiotherapy team must understand how a magnetic field alters the dosimetric properties of the radiation beam and its potential impact on treatment quality and clinical outcomes. This review aims to provide an in-depth description of the magnetic field induced dose effects for the two widely available systems, the 0.35 T and the 1.5 T MR-linacs. In MR-linac treatments, the primary photon beam passes through MR components that never exist in conventional linacs, which alter both in-field and out-of -field doses. More importantly, the interplay between the always-on magnetic field and the secondary electrons is not negligible. This interplay affects dose deposition in the patient, resulting in reduced in-field skin dose due to purged-out contaminant electrons, shortened build-up distance and a shifted crossline profile owing to asymmetric dose kernel. Especially two effects, namely, electron return effect (ERE) and electron stream effect (ESE), are not seen in conventional radiotherapy. This review also summarizes the clinical observations on the site-specific treatments influenced mostly by the magnetic field. In MR-linac treatment, the head and neck region is one of the most challenging sites as ERE occurs at low and high density tissue interfaces and around air cavities, generating hot and cold spots. In breast cancer treatment, consideration should be given to the increased in-field skin dose induced by ERE and the increased out-of -field dose caused by ESE for regions such as the ears, chin, and neck. In lung cancer treatments, tissue inhomogeneity combined with ERE will exacerbate target dose heterogeneity and increase or decrease interface dose. Lastly, treatment in the abdomen and pelvic region will be affected by the presence of gas pockets near the target. The review provides practical recommendations to mitigate these effects.

show abstract

Dosimetry in the presence of strong magnetic fields

Cited by 17 publications

References 21 publications

Measurement validation of treatment planning for a MR‐Linac

Measurement validation of treatment planning for a MR‐Linac

Experimental measurement of ionization chamber angular response and associated magnetic field correction factors in MR‐linac

Magnetic field induced dose effects in radiation therapy using MR‐linacs

Contact Info

Product

Resources

About