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Why do we use margins?
Target / tumor
To
a-priori
compensate for (unknown) deviations between the intended target
position and the real target position during dose delivery
Deviations are
estimated
from population-based measurements of geometrical
errors (can be patient specific, e.g. respiratory motion)
Healthy tissue
To avoid unintended dose to critical organs after aligning the beam to the
displaced target (in case of differential motion between target and OAR)