Dosimetric impact of esophagus motion in single fraction spine stereotactic body radiotherapy.

Nine patients with single fraction spinal stereotactic body radiation therapy (SBRT) treatment were identified to assess both the intra fraction and daily esophageal motion and associated dosimetric deviation in spinal SBRT. We performed a chained deformable registration of 4D CT phase images to estimate the intra fraction motion magnitude of the esophagus in a breathing cycle. The intra fraction esophageal motion mostly exhibited in the superior-inferior direction with the total motion magnitude increased from the T1 (0.7 mm) to the T11 vertebra level (6.5 mm). The actual dose received by a moving esophagus was estimated by accumulating dose from each phase of the 4D dataset using deformable image registration. In comparison, dose recalculated on the average CT reflects the dose received by a stationary esophagus. Intra fraction motion was found to reduce the maximum dose received by a small volume of esophagus  ⩽2 cm3, with the largest absolute and relative dose difference being  -80 cGy and  -6.4%, respectively. Its effect on the maximum dose received by 5 cm3 of esophagus can be higher or lower with a large percentage difference, but did not result in substantial absolute dose increase to violate the dose constraint of 11.9 Gy we used for plan evaluation. In addition, there was no correlation between the dosimetric deviation and the intra fraction motion magnitude. These findings suggest that 4D CT simulation is not essential with regards to the esophageal dose. The daily motion of the esophagus and its dosimetric impact was investigated by examining the difference of esophagus delineated on both treatment and planning CT after they were registered using boney target. The day to day difference of esophagus was negligible for all cases in this study.