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Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams
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Image of FIG. 1.
FIG. 1.

The IBA NACP-02 geometry. Figure 1(a) is the Monte Carlo model of the chamber while Fig. 1(b) is a radiograph of the chamber taken for aid with modeling and to ensure no significant differences between design and manufacture. Major components of the ion chamber are labeled in the radiograph of Fig. 1(b).

Image of FIG. 2.
FIG. 2.

Ion recombination correction factors as a function of dose per pulse for all chamber types collecting positive charge at an operating voltage of 100 V. Linear fits to the data are shown by lines. Most chambers use a plate separation of 2 mm and have very similar gradients in this figure except for the Exradin P11TW (3 mm), the PTW Advanced Markus (1 mm) and the IBA PPC-05 (0.5 mm). In cases where more than one chamber of each type was investigated, a chamber is used that is representative of the chamber type. Error bars representing systematic uncertainties are shown for the Roos chamber.

Image of FIG. 3.
FIG. 3.

Ion recombination correction factors as a function of dose per pulse showing the difference in recombination behavior depending on the polarity of the charge collected for some chambers. Solid symbols (solid lines) are data (linear fits) obtained when positive charge is collected while open symbols (dashed lines) are for negative charge collection. Error bars representing the systematic uncertainty in values are shown for the IBA NACP-02 chamber. Values of at Gy are obtained in cobalt-60 but are not used for the linear fits.

Image of FIG. 4.
FIG. 4.

Beam quality conversion factors with comparison to literature values and Monte Carlo calculations for the subset of chambers for which literature values are available. Filled symbols are calculated factors with a fit [Eq. (5)] to the values shown by the lightly colored line. Present measurements, shifted so that the inside of the chamber window is at the measurement depth and corrected for recombination in cobalt-60, are open circles with error bars representing combined systematic uncertainties. Measured literature values are shown by open squares, connected with straight lines. The dashed lines represent our measurements modified for comparison to literature values as described in the text. In the upper two panels, values are compared to measurements from McEwen et al. (Ref. 1). In the lower two panels, values are compared to factors measured by Kapsch and Gomola (Ref. 2).

Image of FIG. 5.
FIG. 5.

Radiographs of Exradin A11 chambers showing major differences in body construction. Figure 1(a) is the older chamber model (S/N 145) which is no longer available for purchase while Fig. 1(b) shows the new chamber (S/N 81624) which is currently available from Standard Imaging.

Image of FIG. 6.
FIG. 6.

Histograms showing the percent difference between measured and calculated factors for the 6, 10, and 25 MV beams.


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Combined uncertainty in measured coefficients and factors for plane-parallel ion chambers.

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Major dimensions and materials for the plane-parallel ion chambers investigated. The radius of the active region of the chamber is indicated with the total radius (active and guard region) in parentheses. Unless otherwise indicated, the density of graphite used for the Monte Carlo calculations is 1.7 g/cm3. Materials are MYLAR, graphite (Gr), rexolite (cross-linked polystyrene, Rex), polyetheretherketone (PEEK), air-equivalent plastic (C552), polyoxymethylene (POM, trade name Delrin), polystyrene-equivalent plastic (D400), Kapton, and polyethylene (PE). The abbreviation Gr’d refers to a graphited material where a thin layer of graphite is applied to the material in question. Chambers indicated by an asterisk require a water-proofing cap.

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Measured polarity corrections in linac beams and standard deviation.

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Measured recombination parameters with comparison to literature values. Uncertainties on and are estimated to be 17% and 8%, respectively.

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Fitting parameters for Eq. (5) for in terms of %dd(10) x and the rms deviation of the calculated data to the fit. The fit is valid for values of %dd(10) x between 62.7% and 86.1%.

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Measured factors and percent difference between measured and calculated factors. These measured values are those shifted for comparison to Monte Carlo values such that the inside of the front face of the chamber is at the reference depth. Combined uncertainties on measured factors are 0.4%.

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Sample uncertainty budget for Monte Carlo calculated factors for the IBA NACP-02 chamber. Photon cross-sections are assumed to be correlated and contribute a negligible component to the uncertainty in calculated factors.

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Combined uncertainty in Monte Carlo calculated factors. Uncertainty in calculated factors from possible variation in W/e is assumed to be 0.25% as estimated in Sec. III I.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams