Polarity effect in commercial ionization chambers used in photon beams with small fields

Shimono, Tetsunori; Koshida, Kichiro; Nambu, Hidekazu; Matsubara, Kosuke; Takahashi, Hiroshi; Okuda, Hiroshi

doi:10.1007/s12194-008-0050-1

Cited by 16 publications

(17 citation statements)

References 21 publications

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“…Several studies have compared the polarity effect of various ionization chambers. (1,(12)(13)(14) In the present study, the same type of chambers (CC13, IBA Dosimetry) was used for measurement with IBA and Sun Nuclear scanning systems. These chambers showed slight different polarity effects, although the differences were small at shallow region ≤ 19 cm.…”

Section: Discussionmentioning

confidence: 99%

Intra‐ and intervariability in beam data commissioning among water phantom scanning systems

Akino

Gibbons

Neck

et al. 2014

J Applied Clin Med Phys

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Accurate beam data acquisition during commissioning is essential for modeling the treatment planning system and dose calculation in radiotherapy. Although currently several commercial scanning systems are available, there is no report that compared the differences among the systems because most institutions do not acquire several scanning systems due to the high cost, storage space, and infrequent usage. In this report, we demonstrate the intra‐ and intervariability of beam profiles measured with four commercial scanning systems. During a recent educational and training workshop, four different vendors of beam scanning water phantoms were invited to demonstrate the operation and data collection of their systems. Systems were set up utilizing vendor‐recommended protocols and were operated with a senior physicist, who was assigned as an instructor along with vendor. During the training sessions, each group was asked to measure beam parameters, and the intravariability in percent depth dose (PDD). At the end of the day, the profile of one linear accelerator was measured with each system to evaluate intervariability. Relatively very small (SD < 0.12%) intervariability in PDD was observed among four systems at a region deeper than peak (1.5 cm). All systems showed almost identical profiles. At the area within 80% of radiation field, the average, and maximum differences were within ± 0.35% and 0.80%, respectively, compared to arbitrarily chosen IBA system as reference. In the penumbrae region, the distance to agreement (DTA) of the region where dose difference exceed ± 1% was less than 1 mm. Repeated PDD measurement showed small intravariability with SD < 0.5%, although large SD was observed in the buildup region. All four water phantom scanning systems demonstrated adequate accuracy for beam data collection (i.e., within 1% of dose difference or 1 mm of DTA among each other). It is concluded that every system is capable of acquiring accurate beam. Thus the selection of a water scanning system should be based on institutional comfort, personal preference of software and hardware, and financial consideration.PACS number: 87.53.Bn

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Section: Discussionmentioning

confidence: 99%

Intra‐ and intervariability in beam data commissioning among water phantom scanning systems

Akino

Gibbons

Neck

et al. 2014

J Applied Clin Med Phys

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“…The motivations of this work are multifolded. The polarity effect for an ionization chamber is expected to become more dominant with decreasing sensitive air volume . Due to the increasing use of these compact ionization chambers for small field output factor measurements, the need to quantify the dependence of the polarity effect on field sizes of these compact chambers is addressed in this work.…”

Section: Introductionmentioning

confidence: 99%

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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“…In the past, studies of central electrode correction factors have focused on ion chambers with aluminum or graphite electrodes, [7][8][9][10][11] although other investigations have reported strange behavior with ion chambers using high-Z electrodes. [12][13][14][15] Considering the abundance of publications indicating issues with ion chambers that employ high-Z electrodes, there is motivation for a complete study on the effect of the central electrode for these chambers. This work presents a Monte Carlo investigation of the central electrode correction factor, P cel , which is defined as the ratio of the dose to the gas in an ion chamber without the central electrode ͑D ch ͒ to that with the central electrode ͑D ch el ͒,…”

Section: Introductionmentioning

confidence: 99%

The central electrode correction factor for high‐Z electrodes in small ionization chambers

Muir

Rogers

2011

Medical Physics

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In this work, ionization chambers that employ high-Z electrodes have been shown to be problematic in various situations. For beam quality conversion factors, the ratio of Pcel in a beam quality Q to that in a Co-60 beam is required; for some chambers, kQ is significantly different from current dosimetry protocol values because of central electrode effects. It would be best for manufacturers to avoid producing ion chambers that use high-Z electrodes.

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Polarity effect in commercial ionization chambers used in photon beams with small fields

Cited by 16 publications

References 21 publications

Intra‐ and intervariability in beam data commissioning among water phantom scanning systems

Intra‐ and intervariability in beam data commissioning among water phantom scanning systems

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

The central electrode correction factor for high‐Z electrodes in small ionization chambers

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