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°, 3°, and 5° 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 V₂₀). 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 V₂₀ 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 (R² 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° in lung SBRT for medially located tumors. Larger intersubject variations were observed at larger rotational offsets.