A formalism for reference dosimetry in photon beams in the presence of a magnetic field

PurposeReference dosimetry in a strong magnetic field is made more complex due to (a) the change in dose deposition and (b) the change in sensitivity of the detector. Potentially it is also influenced by thin air layers, interfaces between media, relative orientations of field, chamber and radiation, and minor variations in ion chamber stem or electrode construction. The PTW30013 and IBA FC65‐G detectors are waterproof Farmer‐type ion chambers that are suitable for reference dosimetry. The magnetic field correction factors have previously been determined for these chamber types. The aim of this study was to assess the chamber‐to‐chamber variation and determine whether generic chamber type‐specific magnetic field correction factors can be applied for each of the PTW30013 and FC65‐G type ion chambers when they are oriented anti‐parallel (ǁ) to, or perpendicular (⊥) to, the magnetic field.MethodsThe experiment was conducted with 12 PTW30013 and 13 FC65‐G chambers. The magnetic field correction factors were measured using a practical method. In this study each chamber was cross‐calibrated against the local standard chamber twice; with and without magnetic field. Measurements with 1.5 T magnetic field were performed with the 7 MV FFF beam of the MRI‐linac. Measurements without magnetic field (0 T) were performed with the 6 MV conventional beam of an Elekta Agility linac. A prototype MR‐compatible PTW MP1 phantom was used along with a prototype holder that facilitated measurements with the chamber aligned 90° counter‐clockwise (⊥) and 180° (ǁ) to the direction of the magnetic field. A monitor chamber was also mounted on the holder and all measurements were normalized so that the effect of variations in the output of each linac was minimized. Measurements with the local standard chamber were repeated during the experiment to quantify the experimental uncertainty. Recombination was measured in the 6 MV beam. Beam quality correction factors were applied. Differences in recombination and beam quality between beams are constant within each chamber type. By comparing the results for the two cross calibrations the magnetic field correction factors can be determined for each chamber, and the variation within the chamber‐type determined.ResultsThe magnetic field correction factors within both PTW30013 and FC65‐G chamber‐types were found to be very consistent, with observed standard deviations for the PTW30013 of 0.19% (ǁ) and 0.13% (⊥), and for the FC65‐G of 0.15% (ǁ) and 0.17% (⊥). These variations are comparable with the standard uncertainty (k = 1) of 0.24%.ConclusionThe consistency of the results for the PTW30013 and FC65‐G chambers implies that it is not necessary to derive a new factor for every new PTW30013 or FC65‐G chamber. Values for each chamber‐type (with careful attention to beam energy, magnetic field strength and beam‐field‐chamber orientations) can be applied from the literature.

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“…This effect can be incorporated into the reference dosimetry in a number of ways, which are all consistent with:

D_{w, Q, B} = M_{Q, B} \cdot N_{D, w, Q_{0}} \cdot k_{Q, Q_{0}} \cdot k_{B}

…”

Section: Theorymentioning

confidence: 99%

Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors

et al. 2019

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“…The chamber was oriented parallel to the B-field to minimize the influence of the magnetic field on the chamber's response. 8 The effect of the B-field on a chamber similar to the Exradin A28 chamber was evaluated by Malkov and Rogers to be insignificant (<0.2%) when the chamber is aligned parallel to the B-field. 37 During the experiments, the ion chamber was placed at the depth of 8 cm in phantom (i.e., 3 cm underneath the film sample) as a reproducibility check [see Figs.…”

Section: B Irradiation Setupmentioning

confidence: 99%

“…The development of dosimetry for MRIgRT is still at a fairly early stage and it is essential that the performance of detectors used is well characterized. 7 The influence of the B-field on different dosimeters including ionizations chambers, 8,9 plastic scintillators, 10,11 optically stimulated luminescence detectors (OSLDs), 12 thermosluminescent dosimeters (TLDs), 13 diode dosimeters, 14,15 and radiochromic films [16][17][18][19][20][21] have been investigated in recent literature.…”

Section: Introductionmentioning

confidence: 99%

Influence of 0.35 T magnetic field on the response of EBT3 and EBT‐XD radiochromic films

et al. 2020

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To investigate the inconsistency of recent literature on the effect of magnetic field on the response of radiochromic films, we studied the influence of 0.35 T magnetic field on dosimetric response of EBT3 and EBT-XD Gafchromic TM films. Methods: Two different models of radiochromic films, EBT3 and EBT-XD, were investigated. Pieces of films samples from two different batches for each model were irradiated at different dose levels ranging from 1 to 20 Gy using 6 MV flattening filter free (FFF) x-rays generated by a clinical MR-guided radiotherapy system (B = 0.35 T). Film samples from the same batch were irradiated at corresponding dose levels using 6 MV FFF beam from a conventional linac (B = 0) for comparison. The net optical density was measured 48 h postirradiation using a flatbed scanner. The absorbance spectra were also measured over 500-700 nm wavelength range using a fiber-coupled spectrometer with 2.5 nm resolution. To study the effect of fractionated dose delivery to EBT3 (/EBT-XD) films, 8 (/16) Gy dose was delivered in four 2 (/4) Gy fractions with 24 h interval between fractions. Results: No significant difference was found in the net optical density and net absorbance of the films irradiated with or without the presence of magnetic field. No dependency on the orientation of the film during irradiation with respect to the magnetic field was observed. The fractionated dose delivery resulted in the same optical density as delivering the whole dose in a single fraction. Conclusions: The 0.35 T magnetic field employed in the ViewRay ® MR-guided radiotherapy system did not show any significant influence on the response of EBT3 and EBT-XD Gafchromic TM films.

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“…Although no codes of practice specific to MRgRT reference dosimetry have yet been released, several formalisms have been proposed to adapt existing protocols and satisfy this clinical need. 7,[12][13][14][15] Aside from complicating clinical reference dosimetry measurements, there exists an outstanding issue surrounding the use of ICs in solid phantoms: The interaction between the magnetic field and secondary charged particles in the clearance gaps surrounding the chamber that can result in an unpredictable rotational dependence (e.g., rotating the detector about its major axis) on the order of several percent. [16][17][18][19] Clinically, this is of considerable importance, as the use of ICs in solid phantoms is ubiquitous in practice for both machine and patient-specific quality assurance, where time efficiency is as critical as measurement precision.…”

Section: Introductionmentioning

confidence: 99%

Absolute dosimetry of a 1.5 T MR‐guided accelerator‐based high‐energy photon beam in water and solid phantoms using Aerrow

et al. 2020

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Purpose In this work, the fabrication, operation, and evaluation of a probe‐format graphite calorimeter — herein referred to as Aerrow — as an absolute clinical dosimeter of high‐energy photon beams while in the presence of a B = 1.5 T magnetic field is described. Comparable to a cylindrical ionization chamber (IC) in terms of utility and usability, Aerrow has been developed for the purpose of accurately measuring absorbed dose to water in the clinic with a minimum disruption to the existing clinical workflow. To our knowledge, this is the first reported application of graphite calorimetry to magnetic resonance imaging (MRI)‐guided radiotherapy. Methods Based on a previously numerically optimized and experimentally validated design, an Aerrow prototype capable of isothermal operation was constructed in‐house. Graphite‐to‐water dose conversions as well as magnetic field perturbation factors were calculated using Monte Carlo, while heat transfer and mass impurity corrections and uncertainties were assessed analytically. Reference dose measurements were performed in the absence and presence of a B = 1.5 T magnetic field using Aerrow in the 7 MV FFF photon beam of an Elekta MRI‐linac and were directly compared to the results obtained using two calibrated reference‐class IC types. The feasibility of performing solid phantom‐based dosimetry with Aerrow and the possible influence of clearance gaps is also investigated by performing reference‐type dosimetry measurements for multiple rotational positions of the detector and comparing the results to those obtained in water. Results In the absence of the B‐field, as well as in the parallel orientation while in the presence of the B‐field, the absorbed dose to water measured using Aerrow was found to agree within combined uncertainties with those derived from TG‐51 using calibrated reference‐class ICs. Statistically significant differences on the order of (2–4)%, however, were observed when measuring absorbed dose to water using the ICs in the perpendicular orientation in the presence of the B‐field. Aerrow had a peak‐to‐peak response of about 0.5% when rotated within the solid phantom regardless of whether the B‐field was present or not. Conclusions This work describes the successful use of Aerrow as a straightforward means of measuring absolute dose to water for large high‐energy photon fields in the presence of a 1.5 T B‐field to a greater accuracy than currently achievable with ICs. The detector‐phantom air gap does not appear to significantly influence the response of Aerrow in absolute terms, nor does it contribute to its rotational dependence. This work suggests that the accurate use of solid phantoms for absolute point dose measurement is possible with Aerrow.

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A formalism for reference dosimetry in photon beams in the presence of a magnetic field

Cited by 64 publications

References 29 publications

Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors

Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors

Influence of 0.35 T magnetic field on the response of EBT3 and EBT‐XD radiochromic films

Absolute dosimetry of a 1.5 T MR‐guided accelerator‐based high‐energy photon beam in water and solid phantoms using Aerrow

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