LAUSR

Purpose To study the influence of relatively small and uncorrected rotational errors in patient positioning on the dose distribution for Head and Neck and Brain cases. Methods Analysis was carried out for 15 Head and Neck and Brain patients irradiated with VMAT technique. Cone beam CT (CBCT), which was performed for every patient in five to seven fractions during the treatment course, was only used for correcting patient shift. Rotational patient positioning errors were not corrected since it is not possible at most of radiotherapy departments. After the whole course, additional offline registrations of originally acquired CBCT with planning CTs were performed and rotational errors for pitch, roll and yaw (rx, ry, rz) were obtained. Only cases, where errors in patient rotation could not be partially compensated with table shift, were used for this analysis. For every patient, mean rotational corrections were determined. Planning CTs were rotated in all three axes according to the detected mean rotational corrections for individual patients and the original dosimetry plans were recalculated into the rotated CT. The recalculated plans were then used to evaluate the maximum dose to brainstem, optic nerve and chiasma. Results The highest mean rotational positioning errors were found on the X-axis, where the mean rotational error rx was 1,17° (0,20° to 1,98°). For Y and Z-axes the mean rotational errors ry and rz were 0,80° (0,33° to 1,60°) and 0,74° (0,10° to 1,65°), respectively. Recalculated dose distributions in rotated CTs were compared to those in original planning CTs. For all patients maximum doses to brainstem, chiasma and optic nerve were higher in recalculated plans with the mean increase of 0,8 Gy (0,1 to 1,5 Gy), 2,4 Gy (0,3 to 4,3 Gy) and of 2,5 Gy (0,2 to 5,2 Gy), respectively. Conclusions Uncorrected rotational errors for Head and Neck and Brain cases can substantially increase the maximum doses to critical structures. As the dose limits of chiasma (for two patients) and optic nerve (for one patient) were exceeded, the recalculated dose distributions would be evaluated as clinically unacceptable.