Wall correction factors, Pwall, for parallel-plate ionization chambers

Buckley, Lesley; Rogers, D. W. O.

doi:10.1118/1.2199988

Cited by 47 publications

(71 citation statements)

References 24 publications

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“…[3][4][5][6][7][8][9][10][11] A recent Monte Carlo study in ion chamber dosimetry has suggested that in electron beams the wall correction factor ͑P wall ͒ for thimble ion chambers changes with depth by 2.5% 12 and for parallel-plate ion chambers it varies by as much as 6%. 13 However, in ion chamber dosimetry it is commonly assumed that the wall correction factors are unity. Since the uncorrected ion chamber measurements agree with diode measurements, this prompts the question: does the diode response, i.e., the diode reading per unit absorbed dose to the water, change with depth in a manner similar to the ion chamber's P wall correction?…”

Section: Introductionmentioning

confidence: 99%

“…CSnrc is very efficient in calculating dose ratios for similar geometries that are commonly encountered in radiation dosimetry. It has been extensively used in ion chamber dosimetry 12,13 and can be applied to many other applications as well. As an additional verification of the code, we have used CSnrc to calculate the energy and beam quality dependence of the response of LiF TLD chips in mega-voltage photon and electron beams.…”

Section: Introductionmentioning

confidence: 99%

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Monte Carlo study of Si diode response in electron beams

Wang

Rogers

2007

Medical Physics

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Silicon semiconductor diodes measure almost the same depth-dose distributions in both photon and electron beams as those measured by ion chambers. A recent study in ion chamber dosimetry has suggested that the wall correction factor for a parallel-plate ion chamber in electron beams changes with depth by as much as 6%. To investigate diode detector response with respect to depth, a silicon diode model is constructed and the water/silicon dose ratio at various depths in electron beams is calculated using EGSnrc. The results indicate that, for this particular diode model, the diode response per unit water dose (or water/diode dose ratio) in both 6 and 18 MeV electron beams is flat within 2% versus depth, from near the phantom surface to the depth of R50 (with calculation uncertainty <0.3%). This suggests that there must be some other correction factors for ion chambers that counter-balance the large wall correction factor at depth in electron beams. In addition, the beam quality and field-size dependence of the diode model are also calculated. The results show that the water/diode dose ratio remains constant within 2% over the electron energy range from 6 to 18 MeV. The water/diode dose ratio does not depend on field size as long as the incident electron beam is broad and the electron energy is high. However, for a very small beam size (1 X 1 cm(2)) and low electron energy (6 MeV), the water/diode dose ratio may decrease by more than 2% compared to that of a broad beam.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Monte Carlo study of Si diode response in electron beams

Wang

Rogers

2007

Medical Physics

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“…The discrepancy between pl w and h pl corrections is based on the fact that the ratio of P wall pl and P wall w correction factors for planeparallel chambers in Eq. ͑9͒ differs from unity, 8,11 and this is discussed in detail in Secs. IV C and IV D.…”

Section: Ivb Fluence Correction Factormentioning

confidence: 99%

Monte Carlo study of correction factors for the use of plastic phantoms in clinical electron dosimetry

Araki

2007

Medical Physics

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In some recent dosimetry protocols, plastic is allowed as a phantom material for the determination of an absorbed dose to water in electron beams, especially for low energy with beam qualities R50 < 4 g/cm2. In electron dosimetry with plastic, a depth-scaling factor, cpl, and a chamber-dependent fluence correction factor, h(pl), are needed to convert the dose measured at a water-equivalent reference depth in plastic to a dose at a reference depth in water. The purpose of this study is to calculate correction factors for the use of plastic phantoms for clinical electron dosimetry using the EGSnrc Monte Carlo code system. RMI-457 and WE-211 were investigated as phantom materials. First the c(pl) values for plastic materials were calculated as a function of a half-value depth of maximum ionization, I50, in plastic. The c(pl) values for RMI-457 and WE-211 varied from 0.992 to 1.002 and from 0.971 to 0.979, respectively, in a range of nominal energies from 4 MeV to 18 MeV, and varied slightly as a function of I50 in plastic. Since h(pl) values depend on the wall correction factor, P(wall), of the chamber used, they are evaluated using a pure electron fluence correction factor, phi(pl)w, and P(wall)w and P(wall)pl, for a combination of water or plastic phantoms and plane-parallel ionization chambers (NACP-02, Markus and Roos). The phi(pl)w and P(wall) (P(wall)w and P(wall)pl) values were calculated as a function of the water-equivalent depth in plastic materials and at a reference depth as a function of R50 in water, respectively. The phi(pl)w values varied from 1.024 at 4 MeV to 1.013 at 18 MeV for RMI-457, and from 1.025 to 1.016 for WE-211. P(wall)w values for plane-parallel chambers showed values in the order of 1.5% to 2% larger than unity at 4 MeV, consistent with earlier results. The P(wall)pl values of RMI-457 and WE-211 were close to unity for all the energy beams. Finally, calculated h(pl) values of RMI-457 ranged from 1.009 to 1.005, from 1.010 to 1.003 and from 1.011 to 1.007 for NACP-02, Markus and Roos chambers, respectively, in the range of 4 MeV to 18 MeV, and the values of WE-211 were 1.010 to 1.004, 1.010 to 1.004 and 1.012 to 1.008, respectively. The calculated h(pl), values for the Markus chamber agreed within their combined uncertainty with the measured data.

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“…For well-guarded plane-parallel chambers, such as the one recommended in this report, both [77] and McEwen et al [78]). …”

Section: A46 Limiting Value For the Expected Uncertaintymentioning

confidence: 99%

“…[80] and EGSnrc by Verhaegen et al [76] and Buckley and Rogers [77]) and one semiempirical model (by McEwen et al [78]). All of them predict a similar tendency of p wall values slightly above unity at low electron energies and increasing with decreasing energy.…”

Section: A46 Limiting Value For the Expected Uncertaintymentioning

confidence: 99%