PO-0885: Measurement of magnetic field correction factors for multiple radiation detectors for MR linacs

Gohil, Pooja; Agnew, J. P.; Berresford, J.; Budgell, Geoff J; Billas, Ilias; Duane, S

doi:10.1016/s0167-8140(18)31195-2

Cited by 2 publications

(7 citation statements)

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“…However, it can be speculated, that the change in the photon spectrum is more dominant for a change of beam quality from 6 MV to 8 MV (conventional linac), compared to 6 MV (conventional linac) to 7 MV FFF (MR-linac). In this sense, the study of Gohil et al [14] supports our conclusion, that the change of the beam quality between a conventional 6 MV linac and a 7 MV MR-linac does not influence

by more than approximately

.…”

Section: Discussionsupporting

confidence: 84%

“…These results can be compared to the experimental results of Gohil et al [14] . The authors found that

differed by 0.51%, comparing the 6 MV beam quality to the 8 MV beam quality.…”

Section: Discussionmentioning

confidence: 82%

“… [14] did not report any beam quality specifier. Therefore, it cannot be concluded how close the 6 MV and 8 MV beam quality of the conventional linac used by Gohil et al [14] is to the 6 MV beam quality that was used for the Monte Carlo simulations in this work. The same applies the 7 MV FFF beam quality of the 1.5 T MR-linac.…”

Section: Discussionmentioning

confidence: 99%

“…To compensate for this effect, an additional magnetic field correction factor must be applied to the reading of ionization chambers. Recently, several authors have published simulated as well as experimentally determined magnetic field correction factors for Farmer-type ionization chambers [5] , [6] , [7] , [8] , [9] , [10] , [11] , [12] , [13] , [14] . The magnetic field correction factor

can be defined as [13] :

…”

Section: Introductionmentioning

confidence: 99%

“…One approach to determine

was proposed based on experiments that were carried out in setups that combine a conventional 6 MV linac with an electromagnet [5] , [7] , [14] . In such experimental setups, it is not possible to determine

directly.…”

Section: Introductionmentioning

confidence: 99%

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Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Pojtinger

Nachbar

Kapsch

et al. 2020

Physics and Imaging in Radiation Oncology

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Correction factors for reference dosimetry in magnetic resonance (MR) imaging-guided radiation therapy (k) are often determined in setups that combine a conventional 6 MV linac with an electromagnet. This study investigated whether results based on these measurements were applicable for a 7 MV MR-linac using Monte Carlo simulations. For a Farmer-type ionization chamber, k B → ,M,Q was assessed for different tissuephantom ratios (TPR 20,10 ). k B → ,M,Q differed by 0.0029(43) between TPR 20,10 = 0.6790(23)(6 MV linac) and TPR 20,10 = 0.7028(14) (7 MV MR-linac) at 1.5T. The agreement was best in an orientation in which the secondary electrons were deflected to the stem of the ionization chamber.

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by more than approximately

.…”

Section: Discussionsupporting

confidence: 84%

“…These results can be compared to the experimental results of Gohil et al [14] . The authors found that

differed by 0.51%, comparing the 6 MV beam quality to the 8 MV beam quality.…”

Section: Discussionmentioning

confidence: 82%

Section: Discussionmentioning

confidence: 99%

can be defined as [13] :

…”

Section: Introductionmentioning

confidence: 99%

“…One approach to determine

directly.…”

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Pojtinger

Nachbar

Kapsch

et al. 2020

Physics and Imaging in Radiation Oncology

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show abstract

Experimental determination of magnetic field correction factors for ionization chambers in parallel and perpendicular orientations

et al. 2020

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Magnetic field correction factors are needed for absolute dosimetry in magnetic resonance (MR)-linacs. Currently experimental data for magnetic field correction factors, especially for small volume ionization chambers, are largely lacking. The purpose of this work is to establish, independent methods for the experimental determination of magnetic field correction factors k B ⃗ , Q in an orientation in which the ionization chamber is parallel to the magnetic field. The aim is to confirm previous experiments on the determination of Farmer type ionization chamber correction factors and to gather information about the usability of small-volume ionization chambers for absolute dosimetry in MR-linacs. The first approach to determine k B ⃗ , Q is based on a cross-calibration of measurements using a conventional linac with an electromagnet and an MR-linac. The absolute influence of the magnetic field in perpendicular orientation is quantified with the help of the conventional linac and the electromagnet. The correction factors for the parallel orientation are then derived by combining these measurements with relative measurements in the MR-linac. The second technique utilizes alanine electron paramagnetic resonance dosimetry. The alanine system as well as several ionization chambers were directly calibrated with the German primary standard for absorbed dose to water. Magnetic field correction factors for the ionization chambers were determined by a cross-calibration with the alanine in an MR-linac. Important quantities like k B ⃗ , Q for Farmer type ionization chambers in parallel orientation and the change of the dose to water due the magnetic field c B ⃗ have been confirmed. In addition, magnetic field correction factors have been determined for small volume ionization chambers in parallel orientation. The electromagnet-based measurements of k B ⃗ , Q for 7 MV / 1.5 T MR-linacs and parallel ionization chamber orientations resulted in 0.9926(22), 0.9935(31) and 0.9841(27) for the PTW 30013, the PTW 31010 and the PTW 31021, respectively. The measurements based on the second technique resulted in values for k B ⃗ , Q of 0.9901(72), 0.9955(72), and 0.9885(71). Both methods show excellent accuracy and reproducibility and are therefore suitable for the determination of magnetic field correction factors. Small-volume ionization chambers showed a variation in the resulting values for k B ⃗ , Q and should be cross-calibrated instead of using tabulated values for correction factors.

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PO-0885: Measurement of magnetic field correction factors for multiple radiation detectors for MR linacs

Cited by 2 publications

References 0 publications

Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Experimental determination of magnetic field correction factors for ionization chambers in parallel and perpendicular orientations

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