Set-up errors due to endorectal balloon positioning in intensity modulated radiation therapy for prostate cancer

Abstract

Purpose To investigate the set-up errors and deformation associated with daily placement of endorectal balloons in prostate radiotherapy. Materials and methods Endorectal balloons were placed daily in 20 prostate cancer patients undergoing radiotherapy. Electronic portal images (EPIs) were collected weekly from anterior–posterior (AP) and lateral views. The EPIs were compared with digitally reconstructed radiographs from computed tomography scans obtained during pretreatment period to estimate displacements. The interfraction deformation of balloon was estimated with variations in diameter in three orthogonal directions throughout the treatment course. Results A total of 154 EPIs were evaluated. The mean displacements of balloon relative to bony landmark were 1.8 mm in superior–inferior (SI), 1.3 mm in AP, and 0.1 mm in left–right (LR) directions. The systematic errors in SI, AP, and LR directions were 3.3 mm, 4.9 mm, and 4.0 mm, respectively. The random (interfraction) displacements, relative to either bony landmarks or treatment isocenter, were larger in SI direction (4.5 mm and 4.5 mm), than in AP (3.9 mm and 4.4 mm) and LR directions (3.0 mm and 3.0 mm). The random errors of treatment isocenter to bony landmark were 2.3 mm, 3.2 mm, and 2.6 mm in SI, AP, and LR directions, respectively. Over the treatment course, balloon deformations of 2.8 mm, 2.5 mm, and 2.6 mm occurred in SI, AP, and LR directions, respectively. The coefficient of variance of deformation was 7.9%, 4.9%, and 4.9% in these directions. Conclusions Larger interfractional displacement and the most prominent interfractional deformation of endorectal balloon were both in SI direction.