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Summary
The focal spot size (FWHM typically 1-2 mm) influences the photon beam
penumbra width.
Lateral photon beam flattening through a conical flattening filter also
creates additional scatter and increases the off-axis softening effect.
Mean photon energy [MeV] at isocenter roughly equals MV/3, somewhat
lower for high-energy beams.
The geometrical beam alignment is not trivial for rounded leaf edges. It may
vary between accelerator vendors and should be better known among users
and TPS vendors.
Electron beams are strongly influenced by scattering and energy loss
interactions inside the treatment head and depends therefore more on
treatment head design than photons.
Warsaw 2017
57
T Knöös