Monte Carlo study of correction factors for Spencer–Attix cavity theory at photon energies at or above 100 keV

Borg, Jette; Kawrakow, I.; Rogers, D. W. O.; Seuntjens, J

doi:10.1118/1.1287054

Cited by 63 publications

(66 citation statements)

References 28 publications

(45 reference statements)

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“…15 However, the choice of cross sections is unimportant since the analysis that follows depends upon the ratios of responses to theory which, for graphite chambers, has been shown to be independent of the cross sections if a completely consistent set of cross-section data is used. 3,5 We have confirmed that the same holds true for high-Z chambers within the statistical precision considered here. Under the simulation conditions described above, the K h , K an , and K comp corrections are unity by definition.…”

Section: Iia Egsnrc Calculations Of Chamber Responsesupporting

confidence: 82%

Accuracy of Spencer‐Attix cavity theory and calculations of fluence correction factors for the air kerma formalism

Russa¹,

Rogers²

2009

Medical Physics

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EGSnrc calculations of ion chamber response and Spencer-Attix (SA) restricted stopping-power ratios are used to test the assumptions of the SA cavity theory and to assess the accuracy of this theory as it applies to the air kerma formalism for 60Co beams. Consistent with previous reports, the EGSnrc calculations show that the SA cavity theory, as it is normally applied, requires a correction for the perturbation of the charged particle fluence (K(fl)) by the presence of the cavity. The need for K(fl) corrections arises from the fact that the standard prescription for choosing the low-energy threshold delta in the SA restricted stopping-power ratio consistently underestimates the values of delta needed if no perturbation to the fluence is assumed. The use of fluence corrections can be avoided by appropriately choosing delta, but it is not clear how delta can be calculated from first principles. Values of delta required to avoid K(fl) corrections were found to be consistently higher than delta values obtained using the conventional approach and are also observed to be dependent on the composition of the wall in addition to the cavity size. Values of K(fl) have been calculated for many of the graphite-walled ion chambers used by the national metrology institutes around the world and found to be within 0.04% of unity in all cases, with an uncertainty of about 0.02%.

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Section: Iia Egsnrc Calculations Of Chamber Responsesupporting

confidence: 82%

Accuracy of Spencer‐Attix cavity theory and calculations of fluence correction factors for the air kerma formalism

Russa¹,

Rogers²

2009

Medical Physics

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“…Though the electrons below 10 keV contributed to the dose scoring, the stopping power ratios calculated with or without including the track-end effects were agreeable within clinically acceptable range for external photon beams. [28][29][30] Therefore, only the total electron fluence was used in the above stopping-power ratio calculations without explicitly accounting for the track-end effect. The PEGS4 code 18,28 was used to produce the restricted mass-stopping-power ratio between water and the medium, with density corrections based on data from ICRU Report 37.…”

Section: Ratio Of Mass-stopping Powermentioning

confidence: 99%

Variations in energy spectra and water‐to‐material stopping‐power ratios in three‐dimensional conformal and intensity‐modulated photon fields

et al. 2007

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Because of complex dose distributions and dose gradients that are created in three-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiation therapy (IMRT), photon- and electron-energy spectra might change significantly with spatial locations and doses. This study examined variations in photon- and electron-energy spectra in 3D-CRT and IMRT photon fields. The effects of spectral variations on water-to-material stopping-power ratios used in Monte Carlo treatment planning systems and the responses of energy-dependent dosimeters, such as thermoluminescent dosimeters (TLDs) and radiographic films were further studied. The EGSnrc Monte Carlo code was used to simulate megavoltage 3D-CRT and IMRT photon fields. The photon- and electron-energy spectra were calculated in 3D water phantoms and anthropomorphic phantoms based on the fluence scored in voxel grids. We then obtained the water-to-material stopping-power ratios in the local voxels using the Spencer-Attix cavity theory. Changes in the responses of films and TLDs were estimated based on the calculated local energy spectra and published data on the dosimeter energy dependency. Results showed that the photon-energy spectra strongly depended on spatial positions and doses in both the 3D-CRT and IMRT fields. The relative fraction of low-energy photons (< 100 keV) increased inversely with the photon dose in low-dose regions of the fields. A similar but smaller effect was observed for electrons in the phantoms. The maximum variation of the water-to-material stopping-power ratio over the range of calculated dose for both 3D-CRT and IMRT was negligible (< 1.0%) for ICRU tissue, cortical bone, and soft bone and less than 3.6% for dry air and lung. Because of spectral softening at low doses, radiographic films in the phantoms could over-respond to dose by more than 30%, whereas the over-response of TLDs was less than 10%. Thus, spatial variations of the photon- and electron-energy spectra should be considered as important factors in 3D-CRT and IMRT dosimetry.

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“…Measurements made by Henry and reported by Shortt and Ross (1986) indicated the need for such a correction factor. A recent re-examination of the effect using Monte Carlo calculations also indicates the necessity for K comp (Borg et al 2000]). As an interim, K comp is shown explicitly in equation (2) but is assigned the value unity.…”

Section: Uncertainty In the Ratio Of The Absorbed Dose To Water Standmentioning

confidence: 99%

“…In the comparison of air kerma standards of NRC and NIST, it was hypothesized that the observed difference of 0.6% may be due to the different methods of estimating this correction . However, further work at NRC on the value of K comp is continuing (Borg et al 2000) and this may alter the NRC's air kerma standard (Rogers et al 2001).…”

Section: Air Kerma Comparisonmentioning

confidence: 99%

An Analysis of Size Effect on the Performances of Low-Leakage 0.10 µm Complementary Metal Oxide Semiconductor for 5-GHz Band Low-Power RF-ICs and Static Random Access Memory Applications

Lin¹

2004

Jpn. J. Appl. Phys.

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We report the results of a comparison of the dosimetric standards of Canada and France for photon beams at 60 Co and a few higher energies. The present primary standard of absorbed dose to water for NRC, Canada is based on measurements made with a sealed water calorimeter. The corresponding standard of the LNHB, France is based on measurements made with a graphite calorimeter at 60 Co energy and transferred to absorbed dose to water for 60 Co and higherenergy photon beams using both ion chambers and Fricke dosemeters as transfer instruments. To make this comparison, we used three graphite-walled NE2571 Farmer chambers. The absorbed dose to water determined by the LNHB was greater than that determined by NRC by 0.20% at 60 Co energy. This difference is not significant given the uncertainties on the standards. In order to do the comparison for higher-energy photons, we interpolated the NRC data set at the beam qualities used at the LNHB. When %dd(10) x is used as the method of specifying beam quality, the determination of absorbed dose to water by the LNHB is about 0.2% greater than that determined by NRC and consistent with the results at 60 Co. However, when using TPR 20,10 as the beam quality specifier, the LNHB determination is greater than the NRC's determination by 0.8% and 1.2% at 12 and 20 MV respectively. This discrepancy, which systematically increases with increasing energy, eventually exceeds the uncertainties in the ratio of the standards, estimated to be 0.7%. This underscores the importance of selecting the method of specifying beam quality, either %dd(10) x or TPR 20,10 , at least for the 'soft' beams used by NRC in this comparison. In the case of the air kerma standards, which were also compared at 60 Co energy, the LNHB determination was greater than NRC's by 0.14%, which is not significant given the uncertainties on the standards.

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Monte Carlo study of correction factors for Spencer–Attix cavity theory at photon energies at or above 100 keV

Cited by 63 publications

References 28 publications

Accuracy of Spencer‐Attix cavity theory and calculations of fluence correction factors for the air kerma formalism

Accuracy of Spencer‐Attix cavity theory and calculations of fluence correction factors for the air kerma formalism

Variations in energy spectra and water‐to‐material stopping‐power ratios in three‐dimensional conformal and intensity‐modulated photon fields

An Analysis of Size Effect on the Performances of Low-Leakage 0.10 µm Complementary Metal Oxide Semiconductor for 5-GHz Band Low-Power RF-ICs and Static Random Access Memory Applications

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