Correction factors for A1SL ionization chamber dosimetry in TomoTherapy: Machine‐specific, plan‐class, and clinical fields

Gago-Arias, Araceli; Rodríguez-Romero, Ruth; Sánchez-Rubio, P.; González‐Castaño, Diego M.; Gómez, F.; Núñez, Luis A.; Palmans, Hugo; Sharpe, Peter; Pardo-Montero, Juan

doi:10.1118/1.3692181

Cited by 25 publications

(25 citation statements)

References 15 publications

(18 reference statements)

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“…The Exradin A1SL scanning chamber has an active volume of 0.053 cm 3 and an outer diameter of 6.35 mm, and has been previously considered for small field dosimetry . The methods employed in this work mimic the actual methodology employed by clinics to measure point doses in IMRT treatments.…”

Section: Discussionmentioning

confidence: 99%

“…In this study, the response of the Exradin A1SL scanning‐type ionization chamber (Standard Imaging, Middleton, WI) was investigated in 131 clinical delivery sequences through Monte Carlo simulations. Recent work has shown that many microchamber models may not be suitable for reference dosimetry, whereas many mid‐sized scanning chambers (including the Exradin A1SL scanning chamber) are suitable . For this reason, the Exradin A1SL scanning chamber was chosen as a representative model for this work.…”

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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“…E.g. for Tomotherapy, Gago-Arias et al [ 61 ] have calculated a 2 % correction for the A1SL chamber, which agrees with an observed 2 % systematic deviation of absolute dose in beam data measurements taken by the IROC QA Center Houston.…”

Section: Reference and Relative Dosimetrymentioning

confidence: 63%

Planning and Dosimetry for Stereotactic Body Radiotherapy

Dieterich

2014

Stereotactic Body Radiotherapy

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Planning for SBRT is characterized by the need to create high dose gradients from target to critical structures in the vicinity. Since SBRT is a relatively new fi eld, appropriate DVH constraints are still under development in clinical trials and single-center studies.The effect of residual motion such as deformation, rotation and residual respiratory motion has to be considered to determine appropriate ITV and PTV margins. SBRT targets such as lung are located in areas of the body with large tissue inhomogeneities. Treatment algorithms must be capable of modeling these inhomogeneities appropriately to generate a dose calculation which is close to the delivered dose. Planner skill has traditionally played a large role in designing high quality treatment plans with optimum target coverage and tissue sparing. Data mining of existing plans and the use of plan libraries is being explored to aid the treatment planner in standardizing planning approaches to more consistently fi nd the optimum dose distribution. To ensure a close match between plan dose calculations and delivered dose calculations, the dosimetry of small fi elds must be well understood and modelled in the treatment planning system. The small fi elds and sharp dose gradients pose more stringent challenges for patient-specifi c SBRT QA. Keywords IntroductionTreatment planning for stereotactic radiosurgery is very diverse in that it can range from using the same planning techniques as conventional IMRT (Intensity Modulated Radiation Therapy) with the only difference being in the dose-fractionation scheme and the technical constraints of delivery, to

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“…A different approach that has been used relies on combining measured data in the nonstandard machine with data for another machine or generic data from the literature. For To-moTherapy units 13 the field size dependence of TPR 20,10 (s), the tissue phantom ratio at depths of 20 and 10 cm in water for an s × s cm 2 square field [or equivalent field according to BJR Supplement 25 (Ref. 14)] at a constant SDD of 100 cm was observed to be the same as the field size dependence of TPR 20,10 (s) in a different 6 MV photon beam.…”

Section: Introductionmentioning

confidence: 84%

Determination of the beam quality index of high‐energy photon beams under nonstandard reference conditions

Palmans

2012

Medical Physics

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Correction factors for A1SL ionization chamber dosimetry in TomoTherapy: Machine‐specific, plan‐class, and clinical fields

Cited by 25 publications

References 15 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

Planning and Dosimetry for Stereotactic Body Radiotherapy

Determination of the beam quality index of high‐energy photon beams under nonstandard reference conditions

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