Evaluation of Set-Up Errors and Set-Up Margin in Three-Dimensional Conformal Radiotherapy for Pelvic Tumours by Using Electronic Portal Imaging Device

A set-up error is defined as any deviation between the predetermined and actual treatment positions, and is determined by measuring the displacement of the treatment field position by comparing the treatment image to its reference image. Dose distribution in the target volume is dependent on setup margins. It is important to assess the setup errors for each radiotherapy unit to reduce the treatment errors. The aim of this study was to evaluate the systematic and random setup errors using electronic portal imaging device (EPID) for pelvic tumour patients treated by 3D-CRT (conformal radiotherapy) and also to assess set-up margin. 115 pelvic tumour patients were included in this study in which 1150 portal images were assessed. The displacements between DRR (digitally reconstructed radiograph) and the portal images were measured in the direction of right to lateral, superior to inferior in anterior images by matching rigid bony landmarks. Moreover, the displacements between anterior to posterior and superior to inferior were measured in lateral images. The estimated systematic errors were 0.242, 0.255 cm in right to lateral and superior to inferior direction in anterior images, and 0.227, 0.220 cm in anterior to posterior and superior to inferior in lateral images. The estimated random errors were 0.404, 0.367 cm in right to lateral and superior to inferior in anterior images, 0.313, 0.337 cm in anterior to posterior and superior to inferior in lateral images. The determined margins for CTV to PTV based on ICRU were 0.4711, 0.4465, 0.3870, 0.4026 cm in the order of the above mentioned directions. A 0.5 cm safety margin is suggested for all pelvic tumour patients treated with 3D-CRT in Varian 2300CD linear accelerator unit, Apeksha Hospital, Maharagama.