Saturation current and collection efficiency for ionization chambers in pulsed beams

DeBlois, F; Zankowski, Corey; Podgoršak, Ervin B.

doi:10.1118/1.598992

Cited by 42 publications

(44 citation statements)

References 11 publications

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“…Some publications show that this linear dependence is not valid over a certain threshold voltage for various chambers, both plane‐parallel ( 15 , 20 – 23 ) and cylindrical. ( 24 , 25 ) Standards laboratory . The ion recombination is provided by instrument specifications according to the rate in the presence of continuous radiation (

{}^{60}{normalC} o

).…”

Section: Resultsmentioning

confidence: 99%

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Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

Castro

Garcı́a-Vicente

Mínguez

et al. 2008

J Applied Clin Med Phys

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To achieve a good clinical outcome in radiotherapy treatment, a certain accuracy in the dose delivered to the patient is required. Therefore, it is necessary to keep the uncertainty in each of the steps of the process inside some acceptable values, which implies as low a global uncertainty as possible. The work reported here focused on the uncertainty evaluation of absorbed dose to water in the routine calibration for clinical beams in the range of energies used in external‐beam radiotherapy. With this aim, we considered various uncertainty components (corrected electrometer reading, calibration factor, beam quality correction factor, and reference conditions) associated with beam calibration. Results show a typical uncertainty in the determination of absorbed dose to water during beam calibration of approximately 1.3% for photon beams and 1.5% for electron beams (k=1 in both cases) when the ND,w formalism is used and kQ,Q0 is calculated theoretically. These values may vary depending on the uncertainty provided by the standards laboratory for calibration factor, which is shown in the work. For primary standards based on clinical linear accelerator beam energies, the uncertainty in this step of the process could be placed close to 1.0%. We also discuss the possibility of an uncertainty reduction with the adoption of the absorbed dose to water formalism as compared with the air kerma formalism.PACS numbers: 87.53.Dq, 87.53.Hv

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{}^{60}{normalC} o

).…”

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

Castro

Garcı́a-Vicente

Mínguez

et al. 2008

J Applied Clin Med Phys

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“…DeBlois et al (22) have proposed that the two‐voltage method can result in overestimation of saturation current by 0.7%. They have suggested a semi‐empirical model to obtain more accurate measurement of ion recombination and charge multiplication.…”

Section: Discussionmentioning

confidence: 99%

“…In addition, the two‐voltage method for the

P_{ion}

measurement is used as a gold standard in many institutions 10 , 15 during X‐ray beam calibration procedures and the ratio of high and low operating voltage bias, which is at least two, can also be used in this study. Nevertheless, the two‐voltage method can introduce systematic error and a more accurate method can be obtained using the models suggested by DeBlois and Palmans 22 , 23 …”

Section: Discussionmentioning

confidence: 99%

Ion recombination corrections of ionization chambers in flattening filter‐free photon radiation

Wang¹,

Easterling²,

Ting³

2012

J Applied Clin Med Phys

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The flattening filter free (FFF) X‐rays can provide much higher dose rate at the treatment target compared to the conventional flattened X‐rays. However, the substantial increase of dose rate for FFF beams may affect the ion recombination correction factor, which is required for accurate measurements using ionization chambers in clinical dosimetry. The purpose of this work is to investigate the ion recombination of three types of commonly used ion chambers (Farmer, PinPoint and plane‐parallel) in the FFF photon radiation. Both 6 MV and 10 MV flattened and FFF beams were fully commissioned on a Varian TrueBeam linear accelerator. The ion recombination correction factor, normalPion, was determined using the two‐voltage technique for a 0.6 cc Farmer chamber, a 0.015 cc PinPoint chamber, and a 0.02 cc parallel‐plate chamber at different source‐to‐axis distances (SAD) in a solid water phantom or water tank phantom at a depth of 10 cm in a 10×10 cm2 field. Good repeatability of measurements was demonstrated. Less than 1% difference in Pion between the flattened and FFF photons for all three ion chambers was observed. At a SAD of 100 cm and a depth of 10 cm for a 10×10 cm2 field, normalPion for the Farmer chamber was 1.004 and 1.008 for the 6 MV flattened and FFF beams, respectively. At the same setup using the Farmer chamber, normalPion was 1.002 and 1.015 for the 10 MV flattened and FFF beams, respectively. All normalPion results for the Farmer, PinPoint, or parallel plate chamber in the 6 MV and 10 MV flattened and FFF beams were within 2% from the unity (1 ≤ normalPion < 1.02). The normalPion ratio of the FFF to flattened beams was 0.99~1.01 for both 6 MV and 10 MV photons. The ion recombination effect of the Farmer, PinPoint, and plane‐parallel chamber in the FFF beams is not substantially different from that in the conventional flattened beams.PACS number: 87.56.bd

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“…However, in recent years a number of authors (40)(41)(42) have presented recombination data that do not agree with the standard theory. A number of possible mechanisms have been proposed, including ion multiplication, air volume change, and direct collection of primary electrons, but at present there is no consensus as to which, if any, of these is the reason for these anomalous results.…”

Section: Ion Chambersmentioning

confidence: 78%

Radiation Dosimetry for Oncology

McEwen

2006

Encyclopedia of Medical Devices and Instrumentation

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Saturation current and collection efficiency for ionization chambers in pulsed beams

Cited by 42 publications

References 11 publications

Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

Ion recombination corrections of ionization chambers in flattening filter‐free photon radiation

Radiation Dosimetry for Oncology

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