Dosimetric consequences of rotational errors in radiation therapy of pediatric brain tumor patients

Purpose: To quantify the rotational offsets and estimate the dose effect of rotation on the target volume and normal tissues in children with brain tumor. Methods: Twenty-one pediatric patients with brain tumors were included in this study. Cone-beam CT was performed before each treatment and at the end of every other treatment. Translational offsets were corrected before the treatment. An offline analysis was performed to quantify rotational errors. The treatment plans were altered and recalculated to simulate a rotation of 2 degrees and 4 degrees, and the dose changes were quantified. Results: 1016 CBCT datasets were analyzed for this report. The mean of the rotations were not meaningfully different from zero. 18.1% of the fractions had rotations with a magnitude >= 2 degrees, 5.0% had rotations >= 3 degrees and 0.9% had rotations >= 4 degrees. For the 2 degrees rotational simulation, the gEUD values of the PTV and critical structures changed by less than 2%. For the 40 simulation, parallel type normal structures had minor changes (<2%), but serial type normal structures and the PTV had changes of 10% and 5%, respectively. Conclusions: The majority of rotational errors observed were less than 1 degrees. A rotational error of 2 degrees produced negligible changes in the gEUD to critical structures or target volumes. Rotational errors >= 4 degrees produced undesirable results, therefore, at a minimum, errors >2 degrees should be corrected. (C) 2011 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 102 (2012) 206-209