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Origin of peripheral dose contributions
Anders A
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Patient scatter
– Unavoidable
– Decrease from target essentially as:
n
=1 column shaped scatter source
n
=2 “blob” shaped scatter source
Head scatter
Sources are all irradiated parts of the treatment head, main source the flattening filter
Limited by the collimating devices
Machine design dependent (FFF machines have less!)
Leakage, including scatter leakage
Depends on collimator and shield thicknesses and material (density)
Collimator design dependent
Treatment technique dependent
Neutrons
Mainly a high Z phenomena at photon energies above photonuclear threshold values
Avoidable by lower beam energy
Only one factor user controllable on a daily basis, all other once per decade (replacement)!
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