The magnetic field dependent displacement effect and its correction in reference and relative dosimetry

Tekin, Tuba; Blum, Isabel; Delfs, Björn; Schönfeld, Ann-Britt; Poppe, B; Looe, Hui Khee

doi:10.1088/1361-6560/ac4a41

Cited by 3 publications

(3 citation statements)

References 15 publications

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“…Table 1: Diameters of the detectors including wall material and of the detectors sensitive area for all investigated point detectors as well as the sensitive detector volumes in mm 3 as calculated from finite-element methods taken from (Tekin et al, 2020) (*) or calculated using data given in (Tekin et al, 2022) The detector specific K(x) were derived according to Equation 1 from the measured D(x) and M(x) using the iterative van Cittert (1931) deconvolution method as described in Looe et al (2015). The number of iterations was limited to five to suppress the unavoidable noise amplification during the iteration process.…”

Section: Signal Profiles M(x)mentioning

confidence: 99%

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

Kretschmer¹,

Brodbek²,

Looe³

et al. 2022

Phys. Med. Biol.

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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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Section: Signal Profiles M(x)mentioning

confidence: 99%

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

Kretschmer¹,

Brodbek²,

Looe³

et al. 2022

Phys. Med. Biol.

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“…For MR-linac dosimetry, the magnetic field influences charge collection in the air-filled sensitive volume (SV) of the ionization chamber [ 22 ]. For reference dosimetry, near constant correction factors, specific to the magnetic field, can be applied to ionisation chamber measurements beyond d max ; however, in the build-up region correction factors become depth-dependent [ 23 ] due to a loss of charged-particle equilibrium (CPE) conditions. With a variable magnetic field correction factor in the build-up region and a SV thickness in the order of millimetres [ 24 ], the ionisation chamber is not an ideal dosimeter to accurately measure skin dose in a transverse MR-linac.…”

Section: Introductionmentioning

confidence: 99%

High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac

et al. 2023

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The magnetic field of a transverse MR-linac alters electron trajectories as the photon beam transits through materials, causing lower doses at flat entry surfaces and increased doses at flat beam-exiting surfaces. This study investigated the response of a MOSFET detector, known as the MOSkin™, for high-resolution surface and near-surface percentage depth dose measurements on an Elekta Unity. Simulations with Geant4 and the Monaco treatment planning system (TPS), and EBT-3 film measurements, were also performed for comparison. Measured MOSkin™ entry surface doses, relative to Dmax, were (9.9 ± 0.2)%, (10.1 ± 0.3)%, (11.3 ± 0.6)%, (12.9 ± 1.0)%, and (13.4 ± 1.0)% for 1 × 1 cm2, 3 × 3 cm2, 5 × 5 cm2, 10 × 10 cm2, and 22 × 22 cm2 fields, respectively. For the investigated fields, the maximum percent differences of Geant4, TPS, and film doses extrapolated and interpolated to a depth suitable for skin dose assessment at the beam entry, relative to MOSkin™ measurements at an equivalent depth were 1.0%, 2.8%, and 14.3%, respectively, and at a WED of 199.67 mm at the beam exit, 3.2%, 3.7% and 5.7%, respectively. The largest measured increase in exit dose, due to the electron return effect, was 15.4% for the 10 × 10 cm2 field size using the MOSkin™ and 17.9% for the 22 × 22 cm2 field size, using Geant4 calculations. The results presented in the study validate the suitability of the MOSkin™ detector for transverse MR-linac surface dosimetry.

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“…For MR-linac dosimetry, the magnetic eld in uences charge collection in the air-lled sensitive volume (SV) of the ionization chamber [23]. For reference dosimetry, near constant correction factors, speci c to the magnetic eld, can be applied to ionisation chamber measurements beyond d max ; however, in the build-up region correction factors become depth-dependent [24] due to a loss of charged-particle equilibrium (CPE) conditions. With a variable magnetic eld correction factor in the build-up region and a SV thickness in the order of millimetres [25], the ionisation chamber is not an ideal dosimeter to accurately measure skin dose in a transverse MR-linac.…”

Section: Introductionmentioning

confidence: 99%

High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac

Patterson

Stokes

Cutajar

et al. 2022

Preprint

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The magnetic field of a transverse MR-linac alters electron trajectories as the photon beam transits through materials, causing lower doses at flat entry surfaces and increased doses at flat beam-exiting surfaces. This study investigated the response of a MOSFET detector, known as the MOSkin™, for high-resolution surface and near-surface percentage depth dose measurements on an Elekta Unity. Simulations with Geant4 and the Monaco treatment planning system (TPS), and EBT-3 film measurements, were also performed for comparison. Measured MOSkin™ entry surface doses, relative to dmax, were (9.9 ± 0.2) %, (10.1 ± 0.3) %, (11.3 ± 0.6) %, (12.9 ± 1.0) %, and (13.4 ± 1.0) % for 1 × 1 cm2, 3 × 3 cm2, 5 × 5 cm2, 10 × 10 cm2, and 22 × 22 cm2 fields, respectively. Similarly at the beam exit MOSkin™ doses were (37.2 ± 4.9) %, (50.0 ± 2.9) %, (54.9 ± 2.0) %, (63.9 ± 1.6) %, and (62.4 ± 3.0) %. For the investigated fields, the maximum absolute dose differences for Geant4, TPS, and film at the beam entry, relative to MOSkin™ surface doses, were 1.0%, 16.4%, and 24.3%, respectively and at the beam exit, 5.0%, 3.1%, and 5.7%, respectively. The largest increase in exit dose, due to the electron return effect, was 18.0% for the 22 × 22 cm2 field size, using Geant4 calculations. The results presented in the study validate the suitability of the MOSkin™ detector for transverse MR-linac surface dosimetry.

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The magnetic field dependent displacement effect and its correction in reference and relative dosimetry

Cited by 3 publications

References 15 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

High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac

High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac

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