Deriving detector‐specific correction factors for rectangular small fields using a scintillator detector

Qin, Y; Zhong, Hualiang; Snyder, Karen; Huang, Yimei; Chetty, Indrin J.

doi:10.1120/jacmp.v17i6.6433

Cited by 17 publications

(28 citation statements)

References 34 publications

(52 reference statements)

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“…Therefore, the field output factor is simply a dose ratio or the ratio of detector readings multiplied by a Monte Carlo calculated correction factor,

k_{Q_{italicclin}, Q_{italicmsr}}^{f_{italicclin}, f_{italicmsr}}

, which accounts for the difference between the detector response in the clinical and reference field; hence, in small field output factors calculation, these are determined using the equation below:

\frac{D_{w, Q_{italicclin}}^{f_{clin}}}{D_{w, Q_{italicmsr}}^{f_{msr}}} = [\frac{M_{Q_{italicclin}}^{f_{italicclin}}}{M_{Q italicmsr}^{f_{italicmsr}}}] k_{Q_{clin}, Q_{msr}}^{f_{clin}, f_{msr}}

where

M_{Q_{clin}}^{f_{clin}}

is the measured reading for a clinical field size, and

M_{Q msr}^{f_{msr}}

is the measured reading at a reference field size. The magnitude of

k_{Q_{italicclin}, Q_{italicmsr}}^{f_{italicclin}, f_{italicmsr}}

varies widely depending on type of machine (source size), field size, depth of measurement, and more importantly detector design . Mistakes in accounting for these dependencies have produced radiation accidents that have serious consequences for patient care…”

Section: Introductionmentioning

confidence: 99%

Characterization of the plastic scintillation detector Exradin W2 for small field dosimetry

Galavis

Holmes

et al. 2019

Medical Physics

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Purpose Small field dosimetry has been an active area of research for over a decade. It is now known that large dosimetric errors can be introduced if proper detectors or correction factors are not used. The International Atomic Energy Agency (IAEA) through the technical report series No. 483 provides guidelines for small field dosimetry procedures as well as correction factors for most detectors available in the market. The plastic scintillator detector (PSD) Exradin W1 has been found to have a correction factor close to unity; however, it is not designed for beam scanning. To overcome this limitation, the new PSD Exradin W2 has been developed to be used as a scanning as well as a relative dosimeter. Characterization of this detector in small field dosimetry is presented in this study. Methods A 6 MV beam from a Varian‐Edge linac was used to collect data for the characterization of a W2 detector. Cerenkov light ratio (CLR) is corrected through a separate new electrometer system that comes with the W2 detector. The parameters investigated include the dose and dose rate linearity, beam profiles, percent depth dose (PDD), field output factors, and temperature response. The results were compared with Gafchromic film (EBT‐3 film) for beam profiles. The field output factor and temperature response were compared to the Exradin W1 detector. Results The dose linearity measured with 600 MU/min dose rate showed minimal variations (<0.5%) even for small MU, and similar results were seen for dose rate linearity. The comparison of field output factors between the W2 and W1 showed small differences for various depth and field sizes. The temperature response showed small variation when the temperature was varied from 6∘C to 50∘C. The slope was −0.0017false/∘C and −0.0016false/∘C for the W2 and the W1 detector, respectively. The differences in profiles are 0.5% in umbra and penumbra region for 1×1cm2 field size when compared to the EBT‐3 film profile. Conclusions The W2 scintillator detector showed similar dosimetric and temperature properties to the W1 scintillator detector. The main advantage of the W2 detector among other plastic scintillators is the beam scanning capabilities that, combined with its correction factor of 1.0, make it an ideal detector for commissioning of SRS and SBRT techniques.

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“…Therefore, the field output factor is simply a dose ratio or the ratio of detector readings multiplied by a Monte Carlo calculated correction factor,

k_{Q_{italicclin}, Q_{italicmsr}}^{f_{italicclin}, f_{italicmsr}}

, which accounts for the difference between the detector response in the clinical and reference field; hence, in small field output factors calculation, these are determined using the equation below:

\frac{D_{w, Q_{italicclin}}^{f_{clin}}}{D_{w, Q_{italicmsr}}^{f_{msr}}} = [\frac{M_{Q_{italicclin}}^{f_{italicclin}}}{M_{Q italicmsr}^{f_{italicmsr}}}] k_{Q_{clin}, Q_{msr}}^{f_{clin}, f_{msr}}

where

M_{Q_{clin}}^{f_{clin}}

is the measured reading for a clinical field size, and

M_{Q msr}^{f_{msr}}

is the measured reading at a reference field size. The magnitude of

k_{Q_{italicclin}, Q_{italicmsr}}^{f_{italicclin}, f_{italicmsr}}

Section: Introductionmentioning

confidence: 99%

Characterization of the plastic scintillation detector Exradin W2 for small field dosimetry

Galavis

Holmes

et al. 2019

Medical Physics

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“…Response of detectors in small fields and the determination of detector‐specific output correction factors have been extensively investigated for a range of detectors by several research groups, using one of the following three techniques: (a) empirical approach, where uncorrected signal ratios were determined and compared to the field output factors determined with reference detectors, (b) numerical approach, where

k_{Q_{italicclin}, Q_{italicref}}^{f_{italicclin}, f_{italicref}}

were determined with MC simulations, and (c) semi‐empirical approach which combines both, measurements and numerical/analytical calculations, and where

k_{Q_{italicclin}, Q_{italicref}}^{f_{italicclin}, f_{italicref}}

were the most commonly determined through the comparison of measured uncorrected detector's signal ratios with MC calculated field output factors . There are advantages and disadvantages of each of these approaches .…”

Section: Introductionmentioning

confidence: 99%

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams

et al. 2018

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Purpose The goal of this work is to provide a large and consistent set of data for detector‐specific output correction factors, kQitalicclin,Qitalicref0.166667emfitalicclin,fref, for small static fields for seven solid‐state detectors and to determine field output factors, ΩQitalicclin,Qitalicreffclin,fref, using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators and four megavoltage photon beams. Consistent measurement conditions and recommendations given in the International Code of Practice TRS‐483 for small‐field dosimetry were followed throughout the study. Methods ΩQitalicclin,Qitalicreffclin,fref were determined on two linacs, Elekta Versa HD and Varian TrueBeam, for 6 and 10 MV beams with and without flattening filter and for nine fields ranging from 0.5 × 0.5 cm2 to 10 × 10 cm2. Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. Volume averaging correction factors, determined from two‐dimensional (2D) dose matrices obtained with EBT3 films and fitted to bivariate Gaussian function, were used to correct measured signals. kQitalicclin,Qitalicreffclin,fref were determined empirically for six diodes, IBA SFD, IBA Razor, PTW 60008 P, PTW 60012 E, PTW 60018 SRS, and SN EDGE, and a PTW 60019 microDiamond detector. Results Field output factors and detector‐specific kQitalicclin,Qitalicreffclin,fref are presented in the form of analytical functions as well as in the form of discrete values. It is found that in general, for a given linac, small‐field output factors need to be determined for every combination of beam energy and filtration (WFF or FFF) and field size as the differences between them can be statistically significant (P < 0.05). For different beam energies, the present data for kQitalicclin,Qitalicreffclin,fref are found to differ significantly (P < 0.05) from the corresponding data published in TRS‐483 mostly for the smallest fields (<1.5 cm). For the PTW microDiamond detector, statistically significant differences (P < 0.05) between kQitalicclin,Qitalicreffclin,fref values were found for all investigated beams on an Elekta Versa HD linac for field sizes 0.5 × 0.5 cm2 and 0.8 × 0.8 cm2. Significant differences in kQitalicclin,Qitalicreffclin,fref between beams of a given energy but with and without flattening filters are found for measurements made in small fields (<1.5 cm) at a given linac. Differences in kQitalicclin,Qitalicreffclin,fref are also found when measurements are made at different linacs using the same beam energy filtration combination; for the PTW microDiamond detector, these differences were found to be around 6% and were considered as significant. Conclusions Selection of two reference detectors, EBT3 films and W1 plastic scintillator, and use of an analytical function, is a novel approach for the determination of ΩQitalicclin,Qitalicreffclin,fref for small static fields in megavoltage photon beams. Large set of kQitalicclin,Qitalicreffclin,fref data for s...

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“…There are many publications on the dosimetric characteristics of the PSD . Among which, several studies demonstrated that the PSD achieved similar dose response to other widely accepted benchmarks, such as Monte Carlo, Gafchromic films and diamond detectors . The PSD is thus chosen to be the benchmark in our study.…”

Section: Introductionmentioning

confidence: 99%

Technical Note: Evaluation of plastic scintillator detector for small field stereotactic patient‐specific quality assurance

et al. 2017

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Among all three detectors, PSD demonstrated superior performances in plans with small fields and heavy modulation. High consistency and low uncertainty made PSD a suitable detector for clinical routine SRS QA. PinPoint chamber should be avoided for targets smaller than 0.2 cc; film dosimetry can be utilized with careful evaluation of its uncertainty bracket. Compared to PSD measurements, AcurosXB calculation demonstrated high accuracy for nonmodulated small fields. The positive correlation between plan modulation and QA discrepancy calls for our attention for clinical SRS plans with high modulation.

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Deriving detector‐specific correction factors for rectangular small fields using a scintillator detector

Cited by 17 publications

References 34 publications

Characterization of the plastic scintillation detector Exradin W2 for small field dosimetry

Characterization of the plastic scintillation detector Exradin W2 for small field dosimetry

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams

Technical Note: Evaluation of plastic scintillator detector for small field stereotactic patient‐specific quality assurance

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