Estimation of set-up errors and planning target volume margin in craniospinal irradiation: A prospective study

As the set-up inaccuracies increase towards lower spine, a uniform PTV margin risks missing the target or overexposing OARs to radiation, necessitating the need for the differential PTV margin. We calculated the isotropic PTV margin of 8 mm, 12 mm, and 15 mm in the brain, upper spine, and lower spine, respectively, in the absence of daily imaging. The margins can be further reduced using daily image guidance such as CBCT to minimize radiation exposure to surrounding normal tissue.