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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!!