Dosimetric effects of rotational offsets in stereotactic body radiation therapy (SBRT) for lung cancer

To quantitatively evaluate dosimetric effects of rotational offsets in stereotactic body radiation therapy (SBRT) for lung cancer. Overall, 11 lung SBRT patients (8 female and 3 male; mean age: 75.0 years) with medially located tumors were included. Treatment plans with simulated rotational offsets of 1 degrees, 3 degrees, and 5 degrees in roll, yaw, and pitch were generated and compared with the original plans. Both clockwise and counterclockwise rotations were investigated. The following dosimetric metrics were quantitatively evaluated: planning target volume coverage (PTV V-100%), max PTV dose (PTV D-max), percentage prescription dose to 0.35 cc of cord (cord D-0.35 cc), percentage prescription dose to 0.35 cc and 5 cc of esophagus (esophagus D-0.35 cc and D-5 cc), and volume of the lungs receiving at least 20 Gy (lung V-20). Statistical significance was tested using Wilcoxon signed rank test at the significance level of 0.05. Overall, small differences were found in all dosimetric matrices at all rotational offsets: 95.6% of differences were < 1% or < 1 Gy. Of all rotational offsets, largest change in PTV V-100%, PTV D-max, cord D-0.35 cc, esophagus D-0.35 cc, esophagus D-5 cc, and lung V-20 was - 8.36%, - 6.06%, 11.96%, 8.66%, 6.02%, and - 0.69%, respectively. No significant correlation was found between any dosimetric change and tumor-to-cord/esophagus distances (R-2 range: 0 to 0.44). Larger dosimetric changes and intersubject variations were observed at larger rotational offsets. Small dosimetric differences were found owing to rotational offsets up to 5 degrees in lung SBRT for medially located tumors. Larger intersubject variations were observed at larger rotational offsets. (C) 2014 American Association of Medical Dosimetrists.