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

Cranio-spinal irradiation (CSI) planning is complex due to large target volume and numerous organs at risk (OARs). In the index study, we assessed set-up errors and daily shifts during CSI using cone-beam computed tomography (CBCT) on a conventional linear accelerator. The aim of the study is to calculate the planning target volume (PTV) margin for CSI.

CSI was planned by the volumetric modulated arc technique (VMAT). CBCT was taken daily for the first 3 days and twice weekly thereafter at the level of sella, T1, and L3 vertebrae. The CBCT images were coregistered with planning CT images to check daily shifts and to calculate systematic (Σ) and random errors (σ). PTV margins were calculated using Van Herk's (2.5Σ +0.7σ) formula.

A total of 20 medulloblastoma patients were analyzed. In the absence of daily imaging, the required PTV margins along lateral, longitudinal, and vertical directions were 7.8, 7.3, and 7.4 mm (8 mm isotropic margin); 10.1, 11.5, and 8.7 mm (12 mm isotropic margin); and 14.7, 12.4, and 10.6 mm (15 mm isotropic margin) for brain, upper spine, and lower spine, respectively.

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.