Influence of Rotational Setup Error on Tumor Shift in Bony Anatomy Matching Measured with Pulmonary Point Registration in Stereotactic Body Radiotherapy for Early Lung Cancer

To examine the correlation between the patient rotational error measured with pulmonary point registration and tumor shift after bony anatomy matching in stereotactic body radiotherapy for lung cancer. Twenty-six patients with lung cancer who underwent stereotactic body radiotherapy were the subjects. On 104 cone-beam computed tomography measurements performed prior to radiation delivery, rotational setup errors were measured with point registration using pulmonary structures. Translational registration using bony anatomy matching was done and the three-dimensional vector of tumor displacement was measured retrospectively. Correlation among the three-dimensional vector and rotational error and vertebratumor distance was investigated quantitatively. The median and maximum rotational errors of the roll, pitch and yaw were 0.8, 0.9 and 0.5, and 6.0, 4.5 and 2.5, respectively. Bony anatomy matching resulted in a 0.21.6 cm three-dimensional vector of tumor shift. The shift became larger as the vertebratumor distance increased. Multiple regression analysis for the three-dimensional vector indicated that in the case of bony anatomy matching, tumor shifts of 5 and 10 mm were expected for vertebratumor distances of 4.46 and 14.1 cm, respectively. Using pulmonary point registration, it was found that the rotational setup error influences the tumor shift. Bony anatomy matching is not appropriate for hypofractionated stereotactic body radiotherapy with a tight margin.