Evaluation of Patient Set Up Errors in Head and Neck Three-Dimensional (3D) Conformal and Intensity-Modulated Radiotherapy Using Electronic Portal Imaging Device

Interfractional set-up variations may cause deviation of the delivered dose from the planned dose distribution. This study aimed at calculating random and systematic set-up errors using an electronic portal imaging device (EPID) to set the optimum planning target volume (PTV) margins in patients with head and neck cancer who were under treatment with three-dimensional conformal (3DCRT) and intensity-modulated radiotherapy (IMRT).
In this study, 50 patients underwent 3DCRT along with weekly electronic portal image (EPI), and daily IMRT imaging was performed on 50 others. The EPIs were compared with Digitally Reconstructed Radiographs (DRRs) to quantify the systematic, random, and 3D vector length of set-up errors in three translational directions (X, Y, Z). The PTV margins were measured utilizing International Commission on Radiation Units and Measurements report 62, Stroom’s and van Herk’s models.
For 3DCRT and IMRT techniques, the overall mean 3D vector length of displacement was obtained at 3.9 and 2.7 mm, respectively. The maximum systematic and random errors were 1.3 and 1.9 mm for the IMRT technique and 2 and 2.9 mm for 3DCRT, respectively. PTV margins in the three acquisition directions were 2-7.2 mm.
It was found that a 7 mm extension of the clinical target volume (CTV) to PTV margin ensures that 90% of head and neck cancer patients have received 95% of the planned dose.