Small field dosimetry: What have we learnt?

Das, Indra J.; Morales, Johnny; Francescon, P.

doi:10.1063/1.4954111

Cited by 26 publications

(46 citation statements)

References 57 publications

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“…Although it is tempting to assume that a detector sufficiently smaller than a composite field's dose volume is suitable to measure absolute dose in that field, this work has shown that some delivery sequences within a composite field can cause the detector to report the absorbed dose to water erroneously by up to 2.7%. It has been established that detectors misreport the absorbed dose to water at a point in small static fields; however, the presence of a small field or a set of small fields alone as a portion of a composite field does not itself determine whether a detector can be used for a particular delivery. The results determined in this work suggest that for a small volume detector such as the Exradin A1SL scanning chamber, it may be more important to investigate spectral changes and modulations near the detector, rather than only assessing more general quantities such as average MLC‐defined field size.…”

Section: Discussionmentioning

confidence: 99%

“…Anonymized data for 131 unique patient plans treated within the UWCCC network were used for this study. This set of patient data included 92 step‐and‐shoot IMRT plans and 39 VMAT plans generated by Pinnacle v.9.8 treatment planning software (Phillips Radiation Oncology Systems, Fitchburg, WI) using the 6 MV beam of a Varian Clinac ® iX linac (Varian Medical Systems, Palo Alto, CA). Plans were transferred to the University of Wisconsin Medical Radiation Research Center's (UWMRRC) Varian 21EX accelerator for delivery.…”

Section: Methodsmentioning

confidence: 99%

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VMAT and IMRT plan‐specific correction factors for linac‐based ionization chamber dosimetry

et al. 2018

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Purpose The determination of absorbed dose to water from external beam radiotherapy using radiation detectors is currently rooted in calibration protocols that do not account for modulations encountered in patient‐specific deliveries. Detector response in composite clinical fields has not been extensively studied due to the time and effort required to determine these corrections on a case‐by‐case basis. To help bridge this gap in knowledge, corrections for the Exradin A1SL scanning chamber were determined in a large number of composite clinical fields using Monte Carlo methods. The chamber‐specific perturbations that contribute the most to the overall correction factor were also determined. Methods A total of 131 patient deliveries comprised of 834 beams from a Varian C‐arm linear accelerator were converted to EGSnrc Monte Carlo inputs. A validated BEAMnrc 21EX linear accelerator model was used as a particle source throughout the EGSnrc simulations. Composite field dose distributions were compared against a commercial treatment planning system for validation. The simulation geometry consisted of a cylindrically symmetric water‐equivalent phantom with the Exradin A1SL scanning chamber embedded inside. Various chamber perturbation factors were investigated in the egs_chamber user code of EGSnrc and were compared to reference field conditions to determine the plan‐specific correction factor. Results The simulation results indicated that the Exradin A1SL scanning chamber is suitable to use as an absolute dosimeter within a high‐dose and low‐gradient target region in most nonstandard composite fields; however, there are still individual cases that require larger delivery‐specific corrections. The volume averaging and replacement perturbations showed the largest impact on the overall plan‐specific correction factor for the Exradin A1SL scanning chamber, and both volumetric modulated arc therapy (VMAT) and step‐and‐shoot beams demonstrated similar correction factor magnitudes among the data investigated. Total correction magnitudes greater than 2% were required by 9.1% of step‐and‐shoot beams and 14.5% of VMAT beams. When examining full composite plan deliveries as opposed to individual beams, 0.0% of composite step‐and‐shoot plans and 2.6% of composite VMAT plans required correction magnitudes greater than 2%. Conclusions The A1SL scanning chamber was found to be suitable to use for absolute dosimetry in high‐dose and low‐gradient dose regions of composite IMRT plans but even if a composite dose distribution is large compared to the detector used, a correction‐free absorbed dose‐to‐water measurement is not guaranteed.

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

VMAT and IMRT plan‐specific correction factors for linac‐based ionization chamber dosimetry

et al. 2018

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“…This CoP is known as TRS‐483, dosimetry of small static fields used in external beam radiation . Before this CoP, various groups provided initial data for small field measurements . The TRS‐483 provides comprehensive data on every small field detector available in the market at the time and provides correction factors in small fields.…”

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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“…No significant dependence from the energy due to not water equivalence of the Yb-fiber was observed. It is worth noting that the Exradin W1 proved to be a reliable tool for small field dosimetry applications such as RT beams commissioning and quality checks 1,29,30) thanks to its water-equivalence. However, the approach adopted for stem effect correction is very sensitive to the irradiation set up [31][32][33][34] .…”

Section: Resultsmentioning

confidence: 99%

“…Common features of these modern delivery systems are the modulation of the radiation beam, the use of radiation fields of small dimensions 1) suitable for stereotactic treatments and the high conformation of the dose to the target with the consequent possibility of dose escalation and hypo-fractionation approaches 2) .…”

Section: Introductionmentioning

confidence: 99%

Characterization of Yb-doped silica optical fiber as real-time dosimeter

Veronese

Chiodini

Cialdi

et al. 2017

Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XIX

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The near-infrared radioluminescence and dosimetric properties of Yb-doped silica optical fibers, coupled with an optical detector prototype based on an avalanche photo-diode, were studied by irradiating the fibers with clinical beams generated by a Varian Trilogy accelerator. The performances of the system in standard and small field sizes have been also investigated comparing the output factor, percent depth dose and off axis ratio measurements of the prototypal dosimetric system with other commercial sensors.The results demonstrated that the drawback due to the stem effect in Yb-doped silica optical fibers can be managed in a simple but effective way by optical filtering. These features, together with the accuracy and precision achieved by Ybdoped fibers in relative dose assessments make the device promising for in-vivo dosimetry studies in radiation therapy.

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Small field dosimetry: What have we learnt?

Cited by 26 publications

References 57 publications

VMAT and IMRT plan‐specific correction factors for linac‐based ionization chamber dosimetry

VMAT and IMRT plan‐specific correction factors for linac‐based ionization chamber dosimetry

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

Characterization of Yb-doped silica optical fiber as real-time dosimeter

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