Dosimetric Effects of Rotational Setup Error in Volumetric Modulated Arc Radiotherapy on Brain Tumor Patients

Introduction: This study examined the dosimetric effects based on the rotational setup error to correct patient setup errors occur during volumetric modulated arc radiotherapy (VMAT) for brain tumor patients. Material and Methods: This study included 1129 cases of cone beam computed tomography (CBCT) images obtained from 46 brain tumor patients, who experienced VMAT and used the 6DoF (degree of freedom) treatment couch. The dosimetric effects regarding the application of the rotational setup error were examined by comparing the treatment plans. Results: The mean patient setup errors at the lateral (X-axis), longitudinal (Y-axis), and vertical (Z-axis) directions were 0.1±1.4, 0.0±1.1, and -0.4±1.2 mm, respectively. The pitch, roll, and yaw were -0.29±0.61°,-0.42±0.98°, and -0.53±0.69°, respectively. When an absolute value was taken for the setup error, the mean error was 1.06±0.14 mm at the three translation directions, and the error of rotation was 0.82±0.14°, showing a larger error than that of translation. In terms of the mean dose difference by each region of interest (ROI) before and after correcting for the rotational setup error on the treatment plan, Brain_max was 2.17 Gy, and Brain_mean was 0.28 Gy, whereas the maximum and mean of Brain_stem were -3.58 and -4.43 Gy, respectively. These findings suggested a dose difference according to the correction of the rotational setup error. Conclusion: This study indicated that the dose effect is influenced by the rotational setup error in VMAT of brain tumor patients. Moreover, the 6DoF positional correction could reduce the positional uncertainties and deliver a more accurate dose.