Decomposition of the dose conversion factor based on fluence spectra of secondary charged particles: Application to lateral dose profiles in photon fields

Hartmann, Günther H.; Zink, Klemens

doi:10.1002/mp.13081

Cited by 9 publications

(16 citation statements)

References 20 publications

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“…()) and

{\bar{D}}_{m D D, M P 3}

is the dose to the mDD sensitive volume, both in the complete experimental setup. As done in our previous work, 10 and similar to the approach of several authors, 35–39 the factor f was decoupled into two sub‐factors, here named f E (energy dependence correction factor) and f V (volume‐averaging correction factor):

f (r, θ) = f_{E} (r, θ) f_{V} (r, θ) .

…”

Section: Methodsmentioning

confidence: 99%

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate ¹⁹²Ir brachytherapy sources

et al. 2020

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Purpose To investigate the suitability of the microDiamond detector (mDD) type 60019 (PTW‐Freiburg, Germany) to measure the anisotropy function F(r,θ) of High Dose Rate (HDR) 192Ir brachytherapy sources. Methods The HDR 192Ir brachytherapy source, model mHDR‐v2r (Elekta AB, Sweden), was placed inside a water tank within a 4F plastic needle. Four mDDs (mDD1, mDD2, mDD3, and mDD4) were investigated. Each mDD was placed laterally with respect to the source, and measurements were performed at radial distances r = 1 cm, 3 and 5 cm, and polar angles θ from 0° to 168°. The Monte Carlo (MC) system EGSnrc was used to simulate the measurements and to calculate phantom effect, energy dependence and volume‐averaging correction factors. F(r,θ) was determined according to TG‐43 formalism from the detector reading corrected with the MC‐based factors and compared to the consensus anisotropy function CONF(r,θ). Results At 1 cm, the differences between measurements and MC simulations ranged from −0.8% to +0.8% for θ = 0° and from −2.1% to + 2.3% for θ ≠ 0°. At 3 and 5 cm, the differences ranged from +1.4% to +3.9% for θ = 0°, and from −0.4% to +2.9% for θ ≠ 0°. All differences were within the uncertainties (k = 2). At small angles, the phantom effect correction was up to −1.9%. This effect was mainly caused by the air between source and needle tip. The energy correction was angle‐independent everywhere. For small angles at 1 cm, the volume‐averaging correction was up to −2.9% and became less important for larger angles and distances. The differences of the measured F(r,θ) corrected with the MC‐based factors to CONF(r,θ) ranged from –1.0% to +3.4% for mDD1, –2.2% to +4.2% for mDD2, –2.5% to +4.0% for mDD3, and −2.6% to +3.4% for mDD4. All differences were within the uncertainties (k = 2) except one at (3 cm, 0°). For all the mDDs, F(r,0°) was always higher than CONF(r,0°), with average differences of +3.1% (1 cm), +3.6% (3 cm), and +1.9% (5 cm). The inter‐detector variability was within 2.9% (1 cm), 1.8% (3 cm), and 3.4% (5 cm). Conclusions A reproducible method and experimental setup were presented for measuring and validating F(r,θ) of an HDR 192Ir brachytherapy source in a water phantom using the mDD. The phantom effect and the volume‐averaging need to be taken into account, especially for the smaller distances and angles. Good agreement to CONF(r,θ) was obtained. The discrepancies at (1 cm, 0°), accurately predicted by the MC results, may suggest a reconsideration of CONF(r,θ), at least for this position. The slight overestimations at (3 cm,0°) and (5 cm,0°), both in comparison to CONF(r,θ) and MC results, may be due to an underestimation of the air volume between source and needle tip, dark current, intrinsic over‐response of the mDDs, or radiation‐induced charge imbalance in the detector’s components. The results indicate that the mDD is a valuable tool for measurements with HDR 192Ir brachytherapy sources and support its employment for the determination and validation of TG‐43 parameters of such sources.

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“…()) and

{\bar{D}}_{m D D, M P 3}

f (r, θ) = f_{E} (r, θ) f_{V} (r, θ) .

…”

Section: Methodsmentioning

confidence: 99%

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate ¹⁹²Ir brachytherapy sources

et al. 2020

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“…For this factor, an exact mathematical expression can be formulated based on the undisturbed spectral fluence distribution of the charged particles at the point of dose determination and the disturbed spectral fluence distribution averaged over the sensitive volume of the detector positioned at the point of dose determination. As already expressed previously, we consider the subsequent fluence‐based decomposition of the dose conversion factor into detector‐specific perturbation factors as a very useful concept to understand the response of the diamond detector. Factor = the intrinsic response factor reflecting the physical process which leads from the absorbed energy in the detector finally to the detector signal …”

Section: Resultsmentioning

confidence: 99%

“…As already stated in Ref. [], the volume correction factor to be used for the detector cavity filled with the cavity medium depends on both characteristics of the medium, its atomic composition and its density. The density is the dominant medium‐dependent factor and sometimes also referred to as density effect .…”

Section: Discussionmentioning

confidence: 99%

“…Such fluence changes can be quantitatively described by a further fluence‐based decomposition of the dose conversion factor of Eq. () into subfactors . In this work the following decomposition is performed:

f = \frac{\sum φ_{E, i}^{p} \cdot {()(, \frac{L_{Δ}}{normalρ})}_{w, i}}{\sum φ_{E, i}^{p} \cdot {()(, \frac{L_{Δ}}{normalρ})}_{med, i}} \cdot \frac{\sum φ_{E, i}^{p} \cdot {()(, \frac{L_{Δ}}{normalρ})}_{med, i}}{\sum φ_{E, i}^{italiccav} \cdot {()(, \frac{L_{Δ}}{normalρ})}_{med, i}} \cdot \frac{\sum φ_{E, i}^{italiccav} \cdot {()(, \frac{L_{Δ}}{normalρ})}_{med, i}}{\sum φ_{E, i}^{italicdet} \cdot {()(, \frac{L_{Δ}}{normalρ})}_{med, i}}

where the superscript “cav” refers to the bare diamond cavity.…”

Section: Methodsmentioning

confidence: 99%

“…Following this, the cema‐approximated dose conversion factor can be factorized as:

f = s_{w, med}^{SA} \cdot p_{italicint} \cdot p_{italicext}

where the single factors are approximated using the fluence of the secondary electrons only:

s_{w, med}^{SA} = \sum φ_{E, i}^{p} \cdot {(\frac{{normalL}_{normalΔ}}{ρ})}_{normalw, normali} / \sum φ_{E, i}^{p} \cdot {(\frac{{normalL}_{normalΔ}}{ρ})}_{med, normali}

p_{italicint} = \sum φ_{E, i}^{p} \cdot {(\frac{{normalL}_{normalΔ}}{ρ})}_{med, normali} / \sum φ_{E, i}^{cav} \cdot {(\frac{{normalL}_{normalΔ}}{ρ})}_{med, normali}

and

p_{italicext} = \sum φ_{E, i}^{cav} \cdot {(\frac{{normalL}_{normalΔ}}{ρ})}_{med, normali} / \sum φ_{E, i}^{\det} \cdot {(\frac{{normalL}_{normalΔ}}{ρ})}_{med, normali}

This computational approach of a stepwise decomposition is to some extent similar to that outlined by other authors such as, and also to our previous work . In contrast to that, however, in this work only the reduced number of three geometries have been simulated to obtain the perturbation factors

…”

Section: Methodsmentioning

confidence: 99%

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A Monte Carlo study on the PTW 60019 microDiamond detector

Hartmann

Zink

2019

Medical Physics

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Purpose Data on the output correction factor for small photon beam dosimetry of the microDiamond detector manufactured by the company PTW can be found in a variety of papers. Referring either to measurements or to Monte Carlo (MC) calculations, they show substantial disagreements particularly at very small fields. This work reports results of a further MC study aiming at a better understanding of how specific properties of the microDiamond detector are influencing its output correction factor and whether this can explain at least some of the disagreements. Methods In this study the method of a fluence‐based decomposition of the dose conversion factor was used which is considered as a useful tool to understand the response of a detector in nonreference conditions. This decomposition method yields the following three factors: (a) the stopping power ratio water to diamond, (b) a perturbation factor pint taking into account all fluence changes in the transition from a small water voxel at the point of dose determination to the bare diamond detector, and (c) a perturbation factor pext taking into account all additional fluence changes in the fully simulated diamond detector caused by the material and design details outside the sensitive volume. Results Monte Carlo calculated output correction factors were obtained for Co‐60, 6 MV and 10 MV photon beams showing that the maximum variation with field size remained in the order of 2% for quadratic field sizes larger than about 0.3 cm. For field sizes smaller than about 0.5 cm a clear under‐response is obtained at all three radiation qualities in agreement with all known MC calculations, however, in contrast to some measured result. The shape of the output correction factor can be well explained by an opposite mode of action between under‐response expressed by the perturbation factor pint and over‐response expressed by the perturbation factor pext where the first one is mainly influenced by volume averaging, and the second one by a back scatter effect of electrons from the diamond substrate into the sensitive volume. Conclusion The response of microDiamond detector can be well described under various measuring conditions by the dose conversion factor and the dependency of its fluence‐based subfactors on detector characteristics. Monte Carlo simulations offer an improvement in the understanding particularly of small‐field effects by relating the output correction factor to spectral fluence changes in the sensitive volume of the detector. The most significant influence factors are the finite size of the active volume and the presence of the high‐density diamond substrate causing a field size‐dependent backscattering. These perturbations are opposite in their effects. The diamond in the sensitive volume itself and in particular its density has almost no influence. Scattering of results at very small field sizes can be explained by different gradients of dose profiles around the beam axis at identical full width half maximum (FWHM) field size parameters and by possible deviati...

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Cema‐based formalism for the determination of absorbed dose for high‐energy photon beams

et al. 2021

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Purpose Determination of absorbed dose is well established in many dosimetry protocols and considered to be highly reliable using ionization chambers under reference conditions. If dosimetry is performed under other conditions or using other detectors, however, open questions still remain. Such questions frequently refer to appropriate correction factors. A converted energy per mass (cema)‐based approach to formulate such correction factors offers a good understanding of the specific response of a detector for dosimetry under various measuring conditions and thus an estimate of pros and cons of its application. Methods Determination of absorbed dose requires the knowledge of the beam quality correction factor kQ,Qo, where Q denotes the quality of a user beam and Qo is the quality of the radiation used for calibration. In modern Monte Carlo (MC)‐based methods, kQ,Qo is directly derived from the MC‐calculated dose conversion factor, which is the ratio between the absorbed dose at a point of interest in water and the mean absorbed dose in the sensitive volume of an ion chamber. In this work, absorbed dose is approximated by the fundamental quantity cema. This approximation allows the dose conversion factor to be substituted by the cema conversion factor. Subsequently, this factor is decomposed into a product of cema ratios. They are identified as the stopping power ratio water to the material in the sensitive detector volume, and as the correction factor for the fluence perturbation of the secondary charged particles in the detector cavity caused by the presence of the detector. This correction factor is further decomposed with respect to the perturbation caused by the detector cavity and that caused by external detector properties. The cema‐based formalism was subsequently tested by MC calculations of the spectral fluence of the secondary charged particles (electrons and positrons) under various conditions. Results MC calculations demonstrate that considerable fluence perturbation may occur particularly under non‐reference conditions. Cema‐based correction factors to be applied in a 6‐MV beam were obtained for a number of ionization chambers and for three solid‐state detectors. Feasibility was shown at field sizes of 4 × 4 and 0.5 cm × 0.5 cm. Values of the cema ratios resulting from the decomposition of the dose conversion factor can be well correlated with detector response. Under the small field conditions, the internal fluence correction factor of ionization chambers is considerably dependent on volume averaging and thus on the shape and size of the cavity volume. Conclusions The cema approach is particularly useful at non‐reference conditions including when solid‐state detectors are used. Perturbation correction factors can be expressed and evaluated by cema ratios in a comprehensive manner. The cema approach can serve to understand the specific response of a detector for dosimetry to be dependent on (a) radiation quality, (b) detector properties, and (c) electron fluence changes caused by the detecto...

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Decomposition of the dose conversion factor based on fluence spectra of secondary charged particles: Application to lateral dose profiles in photon fields

Cited by 9 publications

References 20 publications

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate ¹⁹²Ir brachytherapy sources

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate ¹⁹²Ir brachytherapy sources

A Monte Carlo study on the PTW 60019 microDiamond detector

Cema‐based formalism for the determination of absorbed dose for high‐energy photon beams

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Decomposition of the dose conversion factor based on fluence spectra of secondary charged particles: Application to lateral dose profiles in photon fields

Cited by 9 publications

References 20 publications

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate 192Ir brachytherapy sources

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate 192Ir brachytherapy sources

A Monte Carlo study on the PTW 60019 microDiamond detector

Cema‐based formalism for the determination of absorbed dose for high‐energy photon beams

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Product

Resources

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Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate ¹⁹²Ir brachytherapy sources

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate ¹⁹²Ir brachytherapy sources