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Origin of peripheral dose contributions
Patient scatter
– Unavoidable
– From AA : Decrease from target essentially as:
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=1 column shaped scatter source
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=2 “blob” shaped scatter source
Head scatter
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Main source the flattening filter, but other parts of linac head too
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Machine design dependent (FFF machines have less)
Leakage, including scatter leakage
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Depends on collimator and shield thicknesses and material (density)
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Collimator design dependent
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Treatment technique dependent
Neutrons
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Mainly a high Z phenomena at photon energies above photonuclear threshold values
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Not considered at all by TPS
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