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그림 1. Changes by roll-rotational setup errors in the maximum dose to the spinal cord (a), oesophagus (c), heart (e) and skin (g) before translational corrections are shown. After translational corrections, changes in the maximum dose to the spinal cord (b), oesophagus (d), heart (f) and skin (h) are also shown.

Abstract
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°, ±2° and ±3°. 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 D95% and V100% of the target volume, D0.35cc of spinal cord, D0.35cc and D5cc of oesophagus and V20Gy 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 V20Gy of lung were -0.3% ± 0.4% and -0.7% ± 2.4%, 1.6 ± 27.9 cGy, -1.6 ± 37.6 cGy, 15.9 ± 25.3 cGy 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.

Author information
​Lee J1,2, Kim JI2,3,4,5, Ye SJ1,3, Kim HJ3,4,6, Carlson J1,2, Park JM2,3,4,5.

1 Program in Biomedical Radiation Sciences, Department of Transdisciplinary Studies, Seoul National University Graduate School of Convergence Science and Technology, Seoul, Korea.
2 Biomedical Research Institute, Seoul National University College of Medicine, Seoul, Korea.
3 Department of Radiation Oncology, Seoul National University Hospital, Seoul, Korea.
4 Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Korea.
5 Center for Convergence Research on Robotics, Advanced Institutes of Convergence Technology, Suwon, Korea.
6 Department of Radiation Oncology, Seoul National University College of Medicine, Seoul, Korea.