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Summary
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The focal spot size (FWHM typically 1-2 mm) influences the photon beam
penumbra width.
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Lateral photon beam flattening through a conical flattening filter also
creates additional scatter and increases the off-axis softening effect.
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Mean photon energy [MeV] at isocenter roughly equals MV/3, somewhat
lower for high-energy beams.
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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.
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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.
Dublin 2018