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How to choose the PTV margin
•
Express required CTV dose for a specified
fraction of patients. For example: 90% of the
patients must get a minimum CTV dose of
95% or more
•
Add first margin so that 90% of the
preparation (systematic) errors are covered
•
Add margin for penumbra and execution
(random) variation so that CTV + first margin
lies within the 95% isodose