The head-scatter factor for small field sizes.
The head-scatter factors H were examined for four different linear accelerators and were found to be similar at field sizes larger than 3 x 3 cm2. Sharply reduced values for small collimator openings were observed for all the accelerators. It is concluded that the head-scatter (or collimator-scatter) factor has two major components. Scatter of photons in various structures in the beam path, especially the flattening filter, causes a slow (about 10%) increase with increased collimator opening. Insertion of a built-in wedge may double this number. When the collimators are closed, they ultimately block photons from the periphery of the source. This may cause a considerable reduction of the primary photon fluence and typically affects fields smaller than 3 x 3 cm2. The effect can be used to estimate the source size, with results that correlate with the design of the bending magnet.
Zhu
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Department of Radiation Oncology, Roger Williams Cancer Center, Brown University, Providence, Rhode Island 02908.
Bjärngard
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D055592:Biophysical Phenomena; D001703:Biophysics; D006801:Humans; D008961:Models, Structural; D008962:Models, Theoretical; D010315:Particle Accelerators; D017785:Photons; D011880:Radiotherapy Planning, Computer-Assisted; D011882:Radiotherapy, High-Energy; D012542:Scattering, Radiation