The output factor correction as function of the photon beam field size – direct measurement and calculation from the lateral dose response functions of gas-filled and solid detectors

Poppinga, Daniela; Delfs, Björn; Meyners, Jutta; Harder, Dietrich; Poppe, Björn; Looe, Hui Khee

doi:10.1016/j.zemedi.2017.07.006

Cited by 33 publications

(73 citation statements)

References 30 publications

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“…The output ratios were measured at square fields with nominal side lengths configured at the accelerator console of 3, 4, 5, 6, 7, 10, 15, 20, 30, and 40 mm. The field output factors were approximated using a commercial scintillation detector Exradin W1 (Standard Imaging, Middleton, USA), which has been demonstrated as a suitable reference detector at small field sizes due to its high water equivalence and small diameter of the active volume (1 mm). W1 measurements were performed using the protocol described in a previous publication for Cerenkov correction by aligning its cylindrical axis to the beam's axis .…”

Section: Methodsmentioning

confidence: 99%

“…The field output factors were approximated using a commercial scintillation detector Exradin W1 (Standard Imaging, Middleton, USA), which has been demonstrated as a suitable reference detector at small field sizes due to its high water equivalence and small diameter of the active volume (1 mm). W1 measurements were performed using the protocol described in a previous publication for Cerenkov correction by aligning its cylindrical axis to the beam's axis . Since measurements were performed on different days, a set of reference output ratios that is associated with the W1 measurements was obtained using a silicon diode detector (Type 60017, PTW‐Freiburg, Germany).…”

Section: Methodsmentioning

confidence: 99%

“…High‐resolution compact detectors are recommended for small field output factor measurements . The most common of these being silicon diodes, or more recently synthetic diamond detector, and commercial scintillation detector . Air‐filled ionization chambers are considered less suitable, which is attributed to (a) the geometrical volume‐averaging effect owing to the physical dimensions of the effective sensitive air cavity and (b) the perturbation of the fluence of secondary electrons due to the low density air cavity and other nonwater equivalent components such as the chamber wall and the inner electrode .…”

Section: Introductionmentioning

confidence: 99%

“…The air cavities of these compact chambers have dimensions in the range comparable to the solid‐state detectors. Field output factor measurements using these compact ionization chambers have been reported . One of the main advantages of these air‐filled ionization chambers is their lower energy dependence compared to the semiconductor counterparts.…”

Section: Introductionmentioning

confidence: 99%

“…A mathematical approach useful to quantify the role of the volume effect in small field dosimetry is the convolution model , represented by the convolution integral

M false(x, y false) = \int_{- \infty}^{+ \infty} \int_{- \infty}^{+ \infty} K false(x - ξ, y - η false) D false(ξ, η false) d ξ d η

which transforms a given lateral dose profile, D ( x , y ), into the distorted lateral signal profile, M ( x , y ), by convolution with the detector's lateral dose–response function, K ( x , y ) . The detector‐specific convolution kernel K ( x , y ) (free from polarity effect) has been shown useful to characterize the physical effects underlying both the small field output correction factors and the distortion of the signal profiles due to the volume effect associated with the detector's construction.…”

Section: Introductionmentioning

confidence: 99%

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The polarity effect of compact ionization chambers used for small field dosimetry

et al. 2018

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Purpose The recent developments of compact air‐filled ionization chambers for use in small photon beams have raised the needs to address the associated polarity effect. The polarity effect of five compact ionization chambers has been quantified at small field sizes. The origins of the polarity effect are studied experimentally and through Monte‐Carlo simulations. For this purpose, the one‐dimensional lateral dose–response functions were determined using positive and negative chamber's polarity. Monte‐Carlo simulations were performed to study the underlying mechanism of the polarity effect by quantifying the charge imbalance in the collecting electrode and cable. Methods Five novel compact ionization chamber designs have been studied (PTW‐Freiburg: Semiflex 3D 31021, PinPoint 3D 31022 and PinPoint 31023; IBA Dosimetry: Razor chamber CC01‐G and Razor Nano‐chamber CC003). Output ratios were measured down to a nominal field side length of 3 mm using both polarities to evaluate the polarity effect at different field sizes. The small field output correction factors were derived using a scintillator detector as reference. To identify the origins of the polarity effect, slit beam measurements were performed to obtain their lateral dose–response functions. All measurements were performed using three chamber orientations: axial, radial crossplane, and radial inplane. The chambers were modeled according to the manufacturers' blueprints using the Monte‐Carlo package EGSnrc. The charge imbalance due to electrons entering and leaving the inner electrode and cable was studied using an adapted user‐code. Results The output ratios obtained using all five chambers show field size‐dependent polarity effects at small field sizes in the axial orientation, whereas no significant field size dependence of the polarity effect has been observed in the radial orientations. The chambers' lateral dose–response functions reveal that the radiation‐induced charge imbalance in the inner electrode and cable is the main cause of the observed polarity effect at small field sizes. The effect is weakest for the largest PTW 31021 chamber but intensifies for smaller chambers with decreasing sensitive air volume. Consistent results have been obtained between Monte‐Carlo simulations and measurement data. Conclusions Awareness needs to be raised so that the polarity effect of novel compact ionization chambers is appropriately accounted for in small field dosimetry. The results in this work are useful to identify the magnitude of the polarity effect correction and to assist in the decisions on choosing the appropriate chambers and setups during measurements. The origins of the observed polarity effect have been elucidated using the chambers' lateral dose–response functions. The adapted Monte‐Carlo user‐code has been used to compute the radiation‐induced charge imbalance in the chamber's components. It opens the possibility to study the chamber's design with the aim to minimize its polarity effect. Small field output correction factors computed according to...

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…A mathematical approach useful to quantify the role of the volume effect in small field dosimetry is the convolution model , represented by the convolution integral

M false(x, y false) = \int_{- \infty}^{+ \infty} \int_{- \infty}^{+ \infty} K false(x - ξ, y - η false) D false(ξ, η false) d ξ d η

Section: Introductionmentioning

confidence: 99%

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The polarity effect of compact ionization chambers used for small field dosimetry

et al. 2018

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Evaluation of the PTW microDiamond in edge‐on orientation for dosimetry in small fields

Brace

Alhujaili

Paino

et al. 2020

J Applied Clin Med Phys

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Purpose: The PTW microDiamond has an enhanced spatial resolution when operated in an edge-on orientation but is not typically utilized in this orientation due to the specifications of the IAEA TRS-483 code of practice for small field dosimetry. In this work the suitability of an edge-on orientation and advantages over the recommended face-on orientation will be presented. Methods: The PTW microDiamond in both orientations was compared on a Varian TrueBeam linac for: machine output factor (OF), percentage depth dose (PDD), and beam profile measurements from 10 × 10 cm 2 to a 0.5 × 0.5 cm 2 field size for 6X and 6FFF beam energies in a water tank. A quantification of the stem effect was performed in edge-on orientation along with tissue to phantom ratio (TPR) measurements. An extensive angular dependence study for the two orientations was also undertaken within two custom PMMA plastic cylindrical phantoms. Results: The OF of the PTW microDiamond in both orientations agrees within 1% down to the 2 × 2 cm 2 field size. The edge-on orientation overresponds in the build-up region but provides improved penumbra and has a maximum observed stem effect of 1%. In the edge-on orientation there is an angular independent response with a maximum of 2% variation down to a 2 × 2 cm 2 field. The PTW microDiamond in edge-on orientation for TPR measurements agreed to the CC01 ionization chamber within 1% for all field sizes. Conclusions: The microDiamond was shown to be suitable for small field dosimetry when operated in edge-on orientation. When edge-on, a significantly reduced angular dependence is observed with no significant stem effect, making it a more versatile QA instrument for rotational delivery techniques.

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Evaluation of the RUBY modular QA phantom for planar and non‐coplanar VMAT and stereotactic radiations

Poppinga¹,

Kretschmer

Brodbek

et al. 2020

J Applied Clin Med Phys

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Purpose This study evaluates the clinical use of the RUBY modular QA phantom for linac QA to validate the integrity of IGRT workflows including the congruence of machine isocenter, imaging isocenter, and room lasers. The results have been benchmarked against those obtained with widely used systems. Additionally, the RUBY phantom has been implemented to perform system QA (End‐to‐End testing) from imaging to radiation for IGRT‐based VMAT and stereotactic radiations at an Elekta Synergy linac. Material and Methods The daily check of IGRT workflow was performed using the RUBY phantom, the Penta‐Guide, and the STEEV phantom. Furthermore, Winston–Lutz tests was carried out with the RUBY phantom and a ball‐bearing phantom to determine the offsets and the diameters of the isospheres of gantry, collimator, and couch rotations, with respect to the room lasers and kV‐imaging isocenter. System QA was performed with the RUBY phantom and STEEV phantom for eight VMAT treatment plans. Additionally, the visibility of the embedded objects within these phantoms in the images and the results of CT and MR image fusions were evaluated. Results All systems used for daily QA of IGRT workflows show comparable results. Calculated shifts based on CBCT imaging agree within 1 mm to the expected values. The results of the Winston–Lutz test based on kV imaging (2D planar and CBCT) or room lasers are consistent regardless of the system tested. The point dose values in the RUBY phantom agree to the expected values calculated using algorithms in Masterplan and Monte Carlo engine in Monaco within 3% of the clinical acceptance criteria. Conclusion All the systems evaluated in this study yielded comparable results in terms of linac QA and system QA procedures. A system QA protocol has been derived using the RUBY phantom to check the IGRT‐based VMAT and stereotactic radiations workflow at an Elekta Synergy linac.

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The output factor correction as function of the photon beam field size – direct measurement and calculation from the lateral dose response functions of gas-filled and solid detectors

Cited by 33 publications

References 30 publications

The polarity effect of compact ionization chambers used for small field dosimetry

The polarity effect of compact ionization chambers used for small field dosimetry

Evaluation of the PTW microDiamond in edge‐on orientation for dosimetry in small fields

Evaluation of the RUBY modular QA phantom for planar and non‐coplanar VMAT and stereotactic radiations

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