The role of the construction and sensitive volume of compact ionization chambers on the magnetic field‐dependent dose response

Delfs, Björn; Blum, Isabel; Tekin, Tuba; Schönfeld, Ann-Britt; Kranzer, Rafael; Poppinga, Daniela; Giesen, U.; Langner, Frank; Kapsch, Ralf‐Peter; Poppe, Björn; Looe, Hui Khee

doi:10.1002/mp.14994

“…Figure1shows the geometry details and material information of the two detectors as implemented in the Monte Carlo code. The sensitive volume of the Semiflex 3D chamber derived using finite-element methods as described inDelfs et al (2021) was implemented. In addition to the 150 MeV protons as used during the experiments, the simulations were also done for incident proton energies of 60 MeV and 240 MeV.…”

mentioning

confidence: 99%

Investigating the lateral dose response functions of point detectors in proton beams

Kretschmer¹,

Brodbek²,

Looe³

et al. 2022

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Objective Point detector measurements in proton fields are perturbed by the volume effect originating from geometrical volume-averaging within the extended detector’s sensitive volume and density perturbations by non-water equivalent detector components. Detector specific lateral dose response functions K(x) can be used to characterize the volume effect within the framework of a mathematical convolution model, where K(x) is the convolution kernel transforming the true dose profile D(x) into the measured signal profile of a detector M(x). The aim of this work is to investigate K(x) for detectors in proton beams. Approach The K(x) for five detectors were determined by iterative deconvolution of measurements of D(x) and M(x) profiles at 2 cm water equivalent depth of a narrow 150 MeV proton beam. Monte Carlo simulations were carried out for two selected detectors to investigate a potential energy dependence, and to study the contribution of volume-averaging and density perturbation to the volume effect. Main results The Monte Carlo simulated and experimentally determined K(x) agree within 2.1% of the maximum value. Further simulations demonstrate that the main contribution to the volume effect is volume-averaging. The results indicate that an energy or depth dependence of K(x) is almost negligible in proton beams. While the signal reduction from a Semiflex 3D ionization chamber in the center of a gaussian shaped field with 2 mm sigma is 32% for photons, it is 15% for protons. When measuring the field with a microDiamond the trend is less pronounced and reversed with a signal reduction for protons of 3.9% and photons of 1.9%. Significance The determined K(x) can be applied to characterize the influence of the volume effect on detectors measured signal profiles at all clinical proton energies and measurement depths. The functions can be used to derive the actual dose distribution from point detector measurements.

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“…Further investigations into correction factors are required as shown by the limited data available on alternative chambers and differences between physically measured and simulated correction factors presented in Table 3 and 4 . Alternative dosimeters to standard reference ion chambers such as smaller volume chambers [ 72 ], microDiamond and microSilicon type detectors [ 73 ] have also been evaluated. Similar to conventional linacs, the output factors for small fields and the response of detectors under non-equilibrium conditions is an emerging area of research [ 74 ].…”

Section: Future Researchmentioning

confidence: 99%

ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators

Begg

¹

,

Jelen

²

,

Moutrie

³

et al. 2023

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Consistency and clear guidelines on dosimetry are essential for accurate and precise dosimetry, to ensure the best patient outcomes and to allow direct dose comparison across different centres. Magnetic Resonance Imaging Linac (MRI-linac) systems have recently been introduced to Australasian clinics. This report provides recommendations on reference dosimetry measurements for MRI-linacs on behalf of the Australiasian College of Physical Scientists and Engineers in Medicine (ACPSEM) MRI-linac working group. There are two configurations considered for MRI-linacs, perpendicular and parallel, referring to the relative direction of the magnetic field and radiation beam, with different impacts on dose deposition in a medium. These recommendations focus on ion chambers which are most commonly used in the clinic for reference dosimetry. Water phantoms must be MR safe or conditional and practical limitations on phantom set-up must be considered. Solid phantoms are not advised for reference dosimetry. For reference dosimetry, IAEA TRS-398 recommendations cannot be followed completely due to physical differences between conventional linac and MRI-linac systems. Manufacturers’ advice on reference conditions should be followed. Beam quality specification of TPR20,10 is recommended. The configuration of the central axis of the ion chamber relative to the magnetic field and radiation beam impacts the chamber response and must be considered carefully. Recommended corrections to delivered dose are $${k}_{{Q}_{msr}{Q}_{0}}^{{f}_{msr}{f}_{ref}}$$ k Q msr Q 0 f msr f ref , a correction for beam quality and $${k}_{\overrightarrow{B},{Q}_{msr}}^{{f}_{msr}}$$ k B → , Q msr f msr , for the impact of the magnetic field on dosimeter response in the magnetic field. Literature based values for $${k}_{\overrightarrow{B},{Q}_{msr}}^{{f}_{msr}}$$ k B → , Q msr f msr are given. It is important to note that this is a developing field and these recommendations should be used together with a review of current literature.

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“…the dose was scored in the active volume given in Figure 3 minus the slice of thickness d. The simple dead volume model from Malkov and Rogers seems to be adequate for the SNC chambers, since the guard ring is flat and fills the entire diameter of the chamber. Delfs et al 8 already investigated the SNC125c chamber and found that this form of guard ring does not produce a dead-volume. As the large SNC600c chamber has a very similar geometry, it was assumed that the simple model for the dead volume geometry is sufficient for the purpose of this study.…”

Section: Dead Volume Effectsmentioning

confidence: 99%

“…[1][2][3] The effect of magnetic fields on the response of ionization chambers has been investigated thoroughly in experimental and Monte Carlo studies on several chamber designs. [4][5][6][7][8][9][10][11] Its magnitude depends on chamber characteristics, such as the (effective) sensitive volume and material composition, but also on the magnetic field strength, the energy spectrum of the incident beam and the chamber orientation with respect to the magnetic field and beam directions. In most cases, large Farmer-type ion chambers were investigated under reference conditions.…”

Section: Introductionmentioning

confidence: 99%

“…They calculated the chamber's response dependence on d for a wide range of ion chambers in external magnetic fields and found a good agreement between their Monte Carlo simulations and experimental data, if the thickness d of the dead volume was between 0.5 and 1 mm. Delfs et al 8 developed a more sophisticated model of the dead volume based on proton microbeam tomography of the effective sensitive volume, i.e. cavity volume minus dead volume, and finite element calculations of the chamber's electric field.…”

Section: Introductionmentioning

confidence: 99%

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Experimental and Monte Carlo‐based determination of magnetic field correction factors kB,Q$k_{B,Q}$ in high‐energy photon fields for two ionization chambers

Alissa

¹

,

Zink

²

,

Kapsch

³

et al. 2023

Medical Physics

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0

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Background: The integration of magnetic resonance tomography into clinical linear accelerators provides high-contrast, real-time imaging during treatment and facilitates online-adaptive workflows in radiation therapy treatments. The associated magnetic field also bends the trajectories of charged particles via the Lorentz force, which may alter the dose distribution in a patient or a phantom and affects the dose response of dosimetry detectors. Purpose: To perform an experimental and Monte Carlo-based determination of correction factors k B,Q , which correct the response of ion chambers in the presence of external magnetic fields in high-energy photon fields. Methods: The response variation of two different types of ion chambers (Sun Nuclear SNC125c and SNC600c) in strong external magnetic fields was investigated experimentally and by Monte Carlo simulations. The experimental data were acquired at the German National Metrology Institute, PTB, using a clinical linear accelerator with a nominal photon energy of 6 MV and an external electromagnet capable of generating magnetic flux densities of up to 1.5 T in opposite directions. The Monte Carlo simulation geometries corresponded to the experimental setup and additionally to the reference conditions of IAEA TRS-398. For the latter, the Monte Carlo simulations were performed with two different photon spectra: the 6 MV spectrum of the linear accelerator used for the experimental data acquisition and a 7 MV spectrum of a commercial MRI-linear accelerator. In each simulation geometry, three different orientations of the external magnetic field, the beam direction and the chamber orientation were investigated. Results: Good agreement was achieved between Monte Carlo simulations and measurements with the SNC125c and SNC600c ionization chambers, with a mean deviation of 0.3% and 0.6%, respectively. The magnitude of the correction factor k B,Q strongly depends on the chamber volume and on the orientation of the chamber axis relative to the external magnetic field and the beam directions.It is greater for the SNC600c chamber with a volume of 0.6 cm 3 than for the SNC125c chamber with a volume of 0.1 cm 3 . When the magnetic field direction and the chamber axis coincide, and they are perpendicular to the beam direction, the ion chambers exhibit a calculated overresponse of less than 0.7(6)% (SNC600c) and 0.3(4)% (SNC125c) at 1.5 T and less than 0.3(0)% (SNC600c) and 0.1(3)% (SNC125c) for 0.35 T for nominal beam energies of 6 MV and 7 MV. This chamber orientation should be preferred, as k B,Q may increase significantlyThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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The role of the construction and sensitive volume of compact ionization chambers on the magnetic field‐dependent dose response

Cited by 12 publications

References 30 publications

Investigating the lateral dose response functions of point detectors in proton beams

Investigating the lateral dose response functions of point detectors in proton beams

ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators

Experimental and Monte Carlo‐based determination of magnetic field correction factors kB,Q$k_{B,Q}$ in high‐energy photon fields for two ionization chambers

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