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Keeping things in perspective
• Margin recipe assumptions
– Perfectly conformal dose distribution
– Large and smooth (compared to penumbra size) CTV
– Translational errors only
– Homogeneous dose distribution
– Large number of fractions (for the 0.7 part)
• Real life
– Not conformal, i.e. margin will depend on shape of dose distribution
– Not smooth
– Lots of changes
translations, rotations, shape changes…
– Inhomogeneous dose distributions
– Any number of fractions (or very few!)
Beware of assumptions!!