Analysis of interfractional set-up errors and intrafractional organ motions during IMRT for head and neck tumors to define an appropriate planning target volume (PTV)- and planning organs at risk volume (PRV)-margins

Background and purposeTo analyze the interfractional set-up errors and intrafractional organ motions and to define appropriate planning target volume (PTV)- and planning organs at risk volume (PRV)-margins in intensity-modulated radiotherapy (IMRT) for head and neck tumors.Patients and methodsTwenty-two patients with head and neck or brain tumors who were treated with IMRT were enrolled. The set-up errors were defined as the displacements of the coordinates of bony landmarks on the beam films from those on the simulation films. The organ motions were determined as the displacements of the coordinates of the landmarks on the images recorded every 3 min for 15 min on the X-ray simulator from those on the initial image.ResultsThe standard deviations (SDs) of the systematic set-up errors (Σ-INTER) and organ motions (Σ-intra) distributed with a range of 0.7–1.3 and 0.2–0.8 mm, respectively. The average of the SDs of the random set-up errors (σ-INTER) and organ motions (σ-intra) ranged from 0.7 to 1.6 mm and from 0.3 to 0.6 mm, respectively. Appropriate PTV-margins and PRV-margins for all the landmarks ranged from 2.0 to 3.6 mm and from 1.8 to 2.4 mm, respectively.ConclusionsWe have adopted a PTV-margin of 5 mm and a PRV-margin of 3 mm for head and neck IMRT at our department.