Setup deviations in wedged pair irradiation of parotid gland and tonsillar tumors, measured with an electronic portal imaging device

The first aim of this study was to quantify estimated translational setup deviations of patients treated with a wedged pair of oblique beams for parotid gland and tonsillar tumors, using portal imaging. The second aim was to design an off-line setup verification procedure, to improve the setup accuracy, if necessary. Thirty-one patients were treated with two conformal fields (anterior-oblique and posterior-oblique). The patients were immobilized with a head cast. For the last 10 patients, the rigidity of the cast was improved while, in addition, wax molds with metal markers were placed into the outer ear for image correlation. Portal images were acquired about weekly. Setup deviations were analyzed, using anatomical structures and, when available, metal markers for image matching. The consistency of the deviations was determined by the correlation between deviations in the cranio-caudal direction, as measured from both beams. When the deviations were consistent, the translational setup deviation during a treatment session could be described by a three-dimensional (3D) vector. A setup verification procedure was designed using a computer simulation. The statistics of the 3D setup deviations were used as input. The output consisted of the resulting setup accuracy and workload (i.e., the number of setup corrections and portal images). Using the anatomical structures for image correlation, the deviations in the cranio-caudal direction were not correlated, either for the old or the improved cast. However, by using the metal markers, the deviations were correlated and a 3D analysis could be performed. The standard deviations, averaged over the three directions, were equal to 1.8 and 1.4 mm for the distribution of systematic and random deviations, respectively. Application of a setup verification procedure, with 0.7 corrections on the average per patient, could potentially reduce the percentage of 3D systematic deviations larger than 4 mm from 30 to 2%. It can be concluded that it was not possible to obtain consistent translational setup deviations, due to rotations. To quantify 3D translational setup deviations, it was necessary to use additional metal markers, which were visible in the portal images of both beams. A further improvement of the setup accuracy is possible by using an off-line setup verification procedure