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°, ±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.