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Variations in energy spectra and water-to-material stopping-power ratios in three-dimensional conformal and intensity-modulated photon fields
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Image of FIG. 1.
FIG. 1.

Differential fluences of photons (a) and (c) and electrons (b) and (d) along the central axis (CAX) and at off-axis of (a) and (b) and (c) and (d) photon beams. Monte Carlo calculations were performed at a depth of for the beams and at for the beams. The field size was in both cases. ⟨E⟩ denotes the mean energy and the asterisk represents the FLURZnrc results. The fluence was normalized to its total value in the voxel [Eq. (4)].

Image of FIG. 2.
FIG. 2.

Differential fluences of photons (a) and electrons (b) in a single IMRT field. The maximum dose point (“in-field”), a low-dose point in the intensity map (“local minimum”), and a beam penumbra dose point (“out-field”) are shown for comparison. The dose profile for the single IMRT field is shown in Fig. 6.

Image of FIG. 3.
FIG. 3.

Differential fluences of photons (a) and electrons (b) in an anthropomorphic phantom irradiated according to a 9-beam composite IMRT plan. The maximum and minimum dose points on the isocenter slice of the phantom are shown for comparison.

Image of FIG. 4.
FIG. 4.

Fluence-weighted mean energies of photons (a) and electrons (b) (open symbols, lower panels) and fractions of low-energy photons and electrons (shaded symbols, upper panels) as a function of normalized measured output for one-field IMRT, composite four-field 3D-CRT, and composite step-and-shoot nine-field IMRT. The measured outputs were obtained from an ion chamber (for the single-field IMRT), and TLDs (for composite 3D and IMRT fields), and were normalized to the maximum measured values for each case.

Image of FIG. 5.
FIG. 5.

Variations in water-to-material stopping-power ratio in a open field with changes in off-axis distance from the central axis at the depth of (source-to-surface distance: ) for (open symbols) and (shaded symbols) photon beams. The water-to-ICRU-tissue stopping power ratio was the same for and beams in this plot.

Image of FIG. 6.
FIG. 6.

Intensity profiles from MC simulation and ion-chamber measurement in a single IMRT beam (shaded symbols, left axis). The results were obtained at a depth in a water phantom with source-to-surface distance. Both the MC and chamber results were normalized to the calibration values in the reference condition ( open field, , beam). Correspondingly, variations in the water-to-material stopping-power ratios (SPR; open symbols, right axis) at various spatial locations or beam intensities were also plotted. The change in SPR was calculated by dividing the SPR at a given point by the corresponding value at the central axis.

Image of FIG. 7.
FIG. 7.

Relative sensitivities of radiographic films and TLDs as a function of normalized measured output for the single IMRT beam (shaded symbols), composite 3D-CRT (open symbols), and composite step-and-shoot IMRT (open symbols) for photon beams. The measured output from chamber or TLDs was normalized to its maximum value for each case. The response of films was estimated using Eq. (3) based on Palm et al. (Ref. 6), and of TLDs was based on Edwards et al. (Table 4 in Ref. 32).


Generic image for table

Water-to-material mass-stopping-power ratios along the central axis and off-axis in and photon beams with a open field.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Variations in energy spectra and water-to-material stopping-power ratios in three-dimensional conformal and intensity-modulated photon fields