Program Information
Dosimetric Impact of Rotational Shift in the Lung SBRT
M Lee , S Mossahebi*, K Prado , F Cifter , B Yi , Univ. of Maryland School Of Medicine, Baltimore, MD
Presentations
SU-I-GPD-T-624 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: A few studies have reported on the application of coach rotation using commercially available systems in setting up lung Stereotactic Body Radiation Therapy (SBRT) patients. This study is to investigate the necessity of rotational shift by considering dosimetric impacts of rotational errors on SBRT.
Methods: Randomly selected 20 lung patients with the lesion size less than 5 cm and treated with SBRT have been used for dosimetric analysis. Three-D dose array calculated from Eclipse has been rotationally shifted (±1˚, ±3˚, ±5˚ for pitch, roll, and yaw) and overlaid to the original CT images. The Dose-Volume histograms of 18-rotational plans were compared to those of the original plan.
Results: No significant dosimetric differences have been observed in the target coverage: D99% and D95% of PTV and conformality index (CI). For all of the cases up to 5˚ of any couch angles dose differences were less than 2.16%, 2.93% for D99 and D95, respectively and variations of CI were also less than 0.05. None of the OAR doses exceeded the dose limit. Maximum differences were 179 cGy, 0.36%, 0.2% for the maximum dose to the heart and V20 of the ipsilateral and the total lungs, respectively. The maximum dose differences of the spinal cords for most cases were less than 150cGy except in a case having 171 and 193 cGy for 3˚ and 5˚ rotation shift, respectively, of which target is located close to the cord.
Conclusion: A method to test the dose variation due to the rotational couch shifts has been suggested and tested. It has been found to be unnecessary to perform rotational shift up to 5˚ for lung SBRT treatments; the translational shift is sufficient for the cases used in this study. This method may be applied and tested after every planning to rule out exceptionally extreme cases.
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