Technical Note: Comprehensive evaluation and implementation of two independent methods for beam monitor calibration for proton scanning beam

Shen, Jiajian; Ding, Xiaoning; Hu, Yanle; Kang, Young Ae; Liu, Wei; Deng, Wei; Bues, Martin

doi:10.1002/mp.13866

Cited by 8 publications

(13 citation statements)

References 21 publications

(76 reference statements)

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“…The observed fluctuations for the Bragg peak chamber's temporal drift prevented the use of a single linear fit for a baseline correction. Recently it was reported that the homogeneity of Bragg peak chambers is not uniform 46,47 and thus effects seen in this study could be influenced by its heterogeneous response. However, as the observed magnetic field dependency was small and the Bragg peak chamber air gap variance was reported to vary by 5%, such an effect would probably not be noticeable.…”

Section: Discussionmentioning

confidence: 78%

MR‐guided proton therapy: Impact of magnetic fields on the detector response

et al. 2021

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Purpose: To investigate the response of detectors for proton dosimetry in the presence of magnetic fields. Material&Methods: Four ionization chambers, two thimble-type and two plane-parallel-type, and a diamond detector were investigated. All detectors were irradiated with homogeneous single-energy-layer fields, using 252.7 MeV proton beams. A Farmer ionization chamber was additionally irradiated in the same geometrical configuration, but with a lower nominal energy of 97.4 MeV. The beams were subjected to magnetic field strengths of 0, 0.25, 0.5, 0.75 and 1 T produced by a research dipole magnet placed at the room's isocenter. Detectors were positioned at 2 cm water-equivalent depth, with their stem perpendicular to both the magnetic field lines and the proton beam's central axis, in the direction of the Lorentz force. Normality and two sample statistical Student's t-tests were performed to assess the influence of the magnetic field on the detectors' responses. Results: For all detectors, a small but significant magnetic-field-dependent change of their response was found. Observed differences compared to the no magnetic field case ranged from +0.5% to-0.7%. The magnetic field dependence was found to be non-linear and highest between 0.25 and 0.5 T for 252.7 MeV proton beams. A different variation of the Farmer chamber response with magnetic field strength was observed for irradiations using lower energy (97.4 MeV) protons. The largest magnetic field effects were observed for plane-parallel ionization chambers. Conclusion: Small magnetic-field-dependent changes in the detector response were identified, which should be corrected for dosimetric applications.

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Section: Discussionmentioning

confidence: 78%

MR‐guided proton therapy: Impact of magnetic fields on the detector response

et al. 2021

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“…A single‐layer field constitutes of a single nominal‐energy beam covering a certain lateral area with equal‐energy spots that are equidistant in both orthogonal directions and in which the same number of particles is delivered per spot. An alternative method using a large‐area plane‐parallel ionization chamber has also been proposed but has not been as well established 5,7–10 . Other efforts have focused on the calculation 11–13 or experimental determination 14–16 of beam quality correction factors at shallow depth.…”

Section: Introductionmentioning

confidence: 99%

“…For the single‐layer method, it has been proposed that a Farmer‐type ionization chamber (IC) or a plane‐parallel ionization chamber (PPIC) can be used for the determination of absorbed dose to water in reference conditions 5,8,10 . Since for most of the clinical proton beam energy range, there are depth dose gradients present at shallow depths, preference may be given to the use of a PPIC.…”

Section: Introductionmentioning

confidence: 99%

“…For the single-layer method, it has been proposed that a Farmer-type ionization chamber (IC) or a plane-parallel ionization chamber (PPIC) can be used for the determination of absorbed dose to water in reference conditions. 5,8,10 Since for most of the clinical proton beam energy range, there are depth dose gradients present at shallow depths, preference may be given to the use of a PPIC. However, there remain indications that the beam quality correction factors for PPIC are not as unique for a given chamber type as they are for most Farmer-type ICs and the long-term stability of commercial PPICs has been shown to be inferior to those of Farmertype ICs.…”

Section: Introductionmentioning

confidence: 99%

“…An alternative method using a largearea plane-parallel ionization chamber has also been proposed but has not been as well established. 5,[7][8][9][10] Other efforts have focused on the calculation [11][12][13] or experimental determination [14][15][16] of beam quality correction factors at shallow depth.…”

Section: Introductionmentioning

confidence: 99%

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Gradient corrections for reference dosimetry using Farmer‐type ionization chambers in single‐layer scanned proton fields

Medin

Trnková

2020

Medical Physics

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Purpose The local depth dose gradient and the displacement correction factor for Farmer‐type ionization chambers are quantified for reference dosimetry at shallow depth in single‐layer scanned proton fields. Method Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5 cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer‐type ionization chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement. Results Depth dose gradients are as low as 0.1–0.4% per mm at measurement depths from 1 to 5 cm in the highest clinical proton energies (with residual ranges higher than 15 cm) and increase to 1% per mm at a residual range of 4 cm and become larger than 3% per mm for residual ranges lower than 2 cm. The literature review shows that the effective point of measurement of Farmer‐type ionization chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer‐type ICs can be used at measurements depths from 1 to 5 cm for which the residual range is 4 cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2 cm and for residual ranges of 15 cm or higher. Conclusion The literature review confirmed that for proton beams the effective point of measurement of Farmer‐type ionization chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer‐type ionization chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4 cm while the thi...

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Long‐term stability of a PTW 34070 large‐area parallel ionization chamber in clinical proton scanning beams

Yamanaka,

Mori,

Matsumoto

et al. 2024

J Applied Clin Med Phys

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PurposeIn the modeling of beam data for proton therapy planning systems, absolute dose measurements are performed utilizing a Bragg peak chamber (BPC), which is a parallel‐plate ionization chamber. The long‐term stability of the BPC is crucial for ensuring accurate absolute dose measurement. The study aims to assess the long‐term stability of the BPC in clinical proton pencil beam scanning delivery.MethodsThe long‐term stability evaluation focused on the BPC‐Type 34070 (PTW Freiburg, Germany), utilizing clinical proton scanning beams from December 2022 to November 2023. Monthly investigations were conducted to evaluate the response and cross‐calibration factor of the BPC and a reference chamber, employing the spread‐out Bragg peak (SOBP) field. Additionally, assessments were made regarding the BPC's response to monoenergetic beams, along with an examination of the impact of polarity and ion recombination on the BPC.ResultsThe response and cross‐calibration factor of the BPC varied up to 1.9% and 1.8%, respectively, while the response of the reference chamber remained within a 0.5% range. The BPC's response to the mono‐energetic beams varied up to 2.0% across all energies, demonstrating similar variation trends in both the SOBP field and mono‐energetic beams. Furthermore, the variations in polarity and ion recombination effect remained stable within a 0.4% range throughout the year. Notably, the reproducibility of the BPC remained high for each measurement conducted, whether for the SOBP field or mono‐energetic beams, with a maximum deviation observed at 0.1%.ConclusionsThe response and cross‐calibration factor of the BPC demonstrated significant variations, with maximum changes of 1.9% and 1.8%, respectively. However, the reproducibility of the BPC remained consistently high for each measurement. It is recommended that when conducting absolute dose measurements using a BPC, its response should be compared and corrected against the reference chamber for each measurement.

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Technical Note: Comprehensive evaluation and implementation of two independent methods for beam monitor calibration for proton scanning beam

Cited by 8 publications

References 21 publications

MR‐guided proton therapy: Impact of magnetic fields on the detector response

MR‐guided proton therapy: Impact of magnetic fields on the detector response

Gradient corrections for reference dosimetry using Farmer‐type ionization chambers in single‐layer scanned proton fields

Long‐term stability of a PTW 34070 large‐area parallel ionization chamber in clinical proton scanning beams

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