Evaluating the Dosimetric Impact of Interfraction Variations During Image-Guided Radiotherapy Using Six-degree-of-freedom Image Registration and Equivalent Uniform Dose Method

This study proposed an off-line method to quantitively analyze the dosimetric impact of daily setup variations on Intensity-modulated radiotherapy (IMRT) when these variations are not corrected due to the lack of image guidance. Series of daily Cone-beam computed tomography (CBCT) images and treatment plan data of a patient with esophageal cancer previously treated using Image-guided radiotherapy (IGRT) were analyzed retrospectively. Daily setup shifts were applied to IMRT plans on a fraction-by-fraction basis using six-degree-of-freedom rigid image registration. The mean interfraction translation displacement was 2.3 ± 1.8, 2.1 ± 0.9, and -1.8 ± 2.6 mm in the LR, AP, and SI directions, respectively. The average interfraction rotation displacement was 2.2 ± 1.2, -0.7 ± 0.8, and -0.7 ± 0.6 degree around the three orthogonal axes. Due to variations, a loss of equivalent uniform dose (EUD) up to 6% for planning target volume (PTV) was noted. Findings confirmed that successful implementation of high precision IMRT required daily image guidance to ensure accurate delivery of sharp gradient dose distributions.