Assessment of Three-Dimensional Set-up Errors in Pelvic Radiation Therapy

Abstract

Set-up errors are the inevitable features of the radiation treatment process. Coverage of target volume is a direct function of set-up margins, which should be optimized to prevent inadvertent irradiation of adjacent normal tissues. The aim of this study is to evaluate three dimensional set-up errors and propose optimum margins for target volume coverage in pelvic radiation therapy. 100 Portal images are obtained for the first two fractions for 50 pelvic cancer patients with Hip Fix thermoplastics. Displacements between Digitally Reconstructed Radiographs (DRR) and Electronic Portal Images (EPI) are estimated and analyzed along major three axes. Mean displacements, population systematic and random errors and three dimensional vectors of displacements are calculated. Set-up margins are calculated using published margin recipes; International Commission on Radiation Units and measurements (ICRU), Stroom and van Herk. The random errors are 0.2339, 0.1603, and 0.1628 cm respectively along antero-posterior (AP), supero-inferior (SI) and medio-lateral (ML) axes. Similarly; the systematic errors are 0.2698, 0.3284, and 0.2568 cm along AP, SI and ML axes respectively. The mean displacement in vertical, longitudinal and lateral axes are 0.103, -0.177, 0.075 cm respectively and standard deviation in vertical, longitudinal and lateral axes are 0.2698, 0.3284, 0.2568 respectively for pelvic cancer. Based on ICRU report 62, the clinical target volume to planning target volume margins are 0.357, 0.365 and 0.304 cm along AP, SI and ML axes respectively. The corresponding values are 0.703, 0.769 and 0.627 cm with Stroom’s formula and 0.838, 0.933 and 0.756 cm with van Herk’s formula. The results suggest that there is a significant difference within the three major axes. Approximately 22% of portal images have exceeded the tolerance limit (>5mm) of pelvic cancer patients with Hip Fix thermoplastics. Calculated set-up margins are compared well with published margin recipes.