Determining P ion , the correction factor for recombination losses in an ionization chamber

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

“…The maximum deviation found in data provided by Weinhous and Meli (26) is 0.1%. The uncertainty of this component is therefore estimated at 0.06%.…”

Section: Resultsmentioning

confidence: 75%

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

“…The ion collection efficiency for several popular commercial ion chambers has been studied in pulsed and continuous photons using Boag's theory (20) . Weinhouse and Meli (15) provided a convenient method for determining

P_{ion}

using the two‐voltage technique with a voltage ratio of two, which was also used in TG‐51 protocol for ion recombination correction. Bruggmoser et al (14) have proposed that the recombination correction factor depends only on dose per pulse and the chamber type, and it is not affected by radiation type or energy.…”

Section: Discussionmentioning

confidence: 99%

“…Although both studies have realized inaccurate estimation of ion recombination using the two‐voltage method, the two‐voltage method for the

P_{ion}

measurement in the TG‐51 practice still can be used in this study because our study is focused on the relative comparison of ion recombination between the FFF and flattened beams. 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%

“…The two‐voltage technique was used to determine P ion for the photon beams 10 , 15 with bias high and low voltages of

{normalV}_{normalH} = 300 V

and

{normalV}_{normalL} = 150 V

. The

{normalP}_{ion}

was computed using the equation from the TG‐51 protocol:

{normalP}_{ion} = (1 ‐ {normalV}_{normalH} / {normalV}_{normalL}) / ({Mraw}^{normalH} / {Mraw}^{normalL} ‐ {normalV}_{normalH} / {normalV}_{normalL}) = 1 / (2 ‐ {Mraw}^{normalH} / {Mraw}^{normalL})

where

Mra w^{H}

and

Mra w^{L}

were ionization readings at the high and low voltage.…”

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

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

Wang¹,

Easterling²,

Ting³

2012

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

“…Charge recombination has been studied extensively in the literature, 3 , 5 , 11 , 12 , 13 , 14 with authors focusing on either flat or nonflat beams. Kry et al (5) has considered both beam types and found the change (

{normalP}_{ion} - 1

) to be two to four times higher for FFF beams than for flat beams.…”

Section: Introductionmentioning

confidence: 99%

P_ion effects in flattening filter‐free radiation beams

Corns¹,

Huang²,

Thomas

2015

Flattening filter‐free radiation beams have higher dose rates that significantly increase the ion recombination rate in an ion chamber's volume and lower the signal read by the chamber‐electrometer pair. The ion collection efficiency correction (Pion) accounts for the loss of signal and subsequently changes dosimetric quantities when applied. We seek to characterize the changes to the percent depth dose, tissue maximum ratio, relative dose factor, absolute dose calibration, off‐axis ratio, and the field width. We measured Pion with the two‐voltage technique and represented Pion as a linear function of the signal strength. This linear fit allows us to correct measurement sets when we have only gathered the high voltage signal and to correct derived quantities. The changes to dosimetric quantities can be up to 1.5%. Charge recombination significantly affects percent depth dose, tissue maximum ratio, and off‐axis ratio, but has minimal impact on the relative dose factor, absolute dose calibration, and field width.PACS number: 87.55.N‐