Scatter factors for a 25-MV x-ray beam.
The scatter factors, i.e., the ratios between the primary and scatter components of absorbed dose, have been determined for a 25-MV x-ray beam. The primary dose is described by attenuation and head-scatter factors. The attenuation coefficient decreased from 0.028 cm-1 at the surface to 0.025 cm-1 at 30 cm depth. The head-scatter factor varied about 8% for collimator settings from 5 x 5 to 40 x 40 cm2. To determine the scatter factors, measured tissue-output ratios were divided by the attenuation and head-scatter factors for the same depth and field size. The results were fitted, with agreement within +/- 1%, to a model which assumes a linear relation between the scatter factor and the field size if the ratio between field size and depth is constant. The model requires transient electron equilibrium, which is present when the depth exceeds 6 cm and the field size 6 x 6 cm2. The scatter factor increases to 1.33 at 30-cm depth and 40 x 40 cm2 field size. The peak scatter factor (PSF) at 3.5-cm depth reaches 1.06 for the largest field. Both PSF and NPSF, the normalized peak scatter factor, are affected by electron disequilibrium, which causes some conceptual ambiguities and numerical uncertainties.
Department of Radiation Oncology, Roger Williams Cancer Center, Brown University, Providence, Rhode Island 02912.
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Bjärngard
D006801:Humans; D010315:Particle Accelerators; D011874:Radiometry / Q000379:methods*; D011882:Radiotherapy, High-Energy; D012542:Scattering, Radiation; D014965:X-Rays