Quantitative ionization chamber alignment to a water surface: Performance of multiple chambers

Ververs, James D.; McEwen, M; Siebers, Jeffrey V.

doi:10.1002/mp.12315

Cited by 5 publications

(6 citation statements)

References 13 publications

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“…We used BEAMnrc from the EGSnrc code system to model treatment heads for Varian Cl2100 and Elekta Precise accelerators. These linacs correspond with those used for measurements in our companion paper . The Varian head model from our previously published description has been updated to include electron contributions from various treatment head shielding components, as indicated by other authors .…”

Section: Monte Carlo Modelmentioning

confidence: 99%

“…These linacs correspond with those used for measurements in our companion paper. 17 The Varian head model from our previously published description [18][19][20] has been updated to include electron contributions from various treatment head shielding components, as indicated by other authors. [21][22][23][24] The Elekta Precise head model has been validated and documented in previous publications.…”

Section: Monte Carlo Modelmentioning

confidence: 99%

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Quantitative ionization chamber alignment to a water surface: Theory and simulation

2017

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The depth-ionization gradient has a discontinuity and is maximized when the outer-radius of a submerged IC coincides with the water surface. This feature can be used to auto-align ICs to the water surface at the time of scanning and/or be applied retrospectively to scan data to quantify absolute IC depth. Utilization of the gCAP should yield accurate and reproducible depth calibration for clinical depth-ionization measurements between setups and between users.

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Section: Monte Carlo Modelmentioning

confidence: 99%

Section: Monte Carlo Modelmentioning

confidence: 99%

Quantitative ionization chamber alignment to a water surface: Theory and simulation

2017

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“…Moreover, it has been stated that the experimental determination of EPOMs is difficult and that a positioning precision of 0.1 mm is necessary to compare different thimble chambers . Positioning detectors at the water surface based on reflection images may introduce systematic errors and the surface tension of the water may obscure the initial depth setting, especially for the plane‐parallel chambers. Therefore, for the plane‐parallel chambers, the initial depth was carefully set and verified with a 3D‐printed, light plastic cone (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…Siebers et al . and Ververs et al proposed calculating the first derivative of the depth dose curve and finding the maximum. This point indicates the position when the wall of a cylindrical detector just reaches the water surface.…”

Section: Discussionmentioning

confidence: 99%

The effective point of measurement for depth‐dose measurements in small MV photon beams with different detectors

Wegener

Sauer

2019

Medical Physics

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Purpose: The effective point of measurement (EPOM) of cylindrical ionization chambers differs from their geometric center. The exact shift depends on chamber construction details, above all the chamber size, and to some degree on the field-size and beam quality. It generally decreases as the chamber dimensions get smaller. In this work, effective points of measurement in small photon fields of a range of cylindrical chambers of different sizes are investigated, including small chambers that have not been studied previously. Methods: In this investigation, effective points of measurement for different ionization chambers (Farmer type, scanning chambers, micro-ionization chambers) and solid state detectors were determined by measuring depth-ionization curves in a 6 MV beam in field sizes between 2 9 2 cm 2 and 10 9 10 cm 2 and comparing those curves with curves measured with plane-parallel chambers. Results: It was possible to average the results to one shift per detector, as the results were sufficiently independent of the studied field sizes. For cylindrical ion chambers, shifts of the EPOM were determined to be between 0.49 and 0.30 times the inner chamber radius from the reference point. Conclusions:We experimentally confirmed the previously reported decrease of the EPOM shift with decreasing detector size. Highly accurate data for a large range of detectors, including new very small ones, were determined. Thus, small chambers noticeably differ from the 0.5-times to 0.6-times the inner chamber radius recommendations in current dosimetry protocols. The detector-individual EPOMs need to be considered for measurements of depth-dose curves.

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“…To position the IC at the initial depth the most accurate method is to use radiation measurements to localize the surface of the water as the IC is scanned in small steps through the water surface. 10,11 With this method, the surface can be localized with an uncertainty of 0.15 mm.…”

Section: Positioning the Ionization Chamber Along The Beam Axismentioning

confidence: 99%

AAPM WGTG51 Report 374: Guidance for TG‐51 reference dosimetry

Muir

Culberson

Davis³

et al. 2022

Medical Physics

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Practical guidelines that are not explicit in the TG‐51 protocol and its Addendum for photon beam dosimetry are presented for the implementation of the TG‐51 protocol for reference dosimetry of external high‐energy photon and electron beams. These guidelines pertain to: (i) measurement of depth‐ionization curves required to obtain beam quality specifiers for the selection of beam quality conversion factors, (ii) considerations for the dosimetry system and specifications of a reference‐class ionization chamber, (iii) commissioning a dosimetry system and frequency of measurements, (iv) positioning/aligning the water tank and ionization chamber for depth ionization and reference dose measurements, (v) requirements for ancillary equipment needed to measure charge (triaxial cables and electrometers) and to correct for environmental conditions, and (vi) translation from dose at the reference depth to that at the depth required by the treatment planning system. Procedures are identified to achieve the most accurate results (errors up to 8% have been observed) and, where applicable, a commonly used simplified procedure is described and the impact on reference dosimetry measurements is discussed so that the medical physicist can be informed on where to allocate resources.

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Quantitative ionization chamber alignment to a water surface: Performance of multiple chambers

Cited by 5 publications

References 13 publications

Quantitative ionization chamber alignment to a water surface: Theory and simulation

Quantitative ionization chamber alignment to a water surface: Theory and simulation

The effective point of measurement for depth‐dose measurements in small MV photon beams with different detectors

AAPM WGTG51 Report 374: Guidance for TG‐51 reference dosimetry

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