Dosimetric effects of roll rotational setup errors on lung stereotactic ablative radiotherapy using volumetric modulated arc therapy

Objective: To evaluate the dosimetric effects of roll-rotational setup errors of stereotactic ablative radiotherapy (SABR) for lung cancer using volumetric modulated arc therapy (VMAT). Methods: A total of 23 lung SABR cases were evaluated retrospectively. Each of the planning CT images was intentionally rotated by +/- 1 degrees, +/- 2 degrees and +/- 3 degrees. After that, to simulate the translational couch correction, rotated CT images were moved along the x, y and z axis to match the centroid of the target volume in the rotated CT images with that in the original CT images. The differences in D-95% and V-100% of the target volume, D-0.35cc of spinal cord, D-0.35cc and D-5cc of oesophagus and V-20Gy of lung between the original and the rotated CT images were calculated. Results: The average differences in D95% and V100% of target volume, D0.35cc of spinal cord, D0.35cc and D5cc of oesophagus and V-20Gy of lung were -0.3% +/- 0.4% and -0.7% +/- 2.4%, 1.6 +/- 27.9 cGy, -1.6 +/- 37.6cGy, 15.9 +/- 25.3cGy and 0.0%+/- 0.1%, respectively. The dosimetric changes in organs at risk (OARs) near the target volume were sometimes considerable due to roll-rotational setup errors, despite the translational correction, and those were patient specific. Conclusion: In the case of coplanar VMAT for lung SABR, dosimetric changes to the target volume due to roll-rotational setup errors could be compensated by translational correction, whereas those to the OARs could not in some cases. Advances in knowledge: Roll-rotational setup errors would increase the dose to OARs despite the translational correction.