3-DIMENSIONAL VARIABILITY IN PATIENT POSITIONING USING BITE BLOCK IMMOBILIZATION IN 3D-CONFORMAL RADIATION TREATMENT FOR ENT-TUMORS

Background and purpose: The aim of this prospective study was to analy ze, the three-dimensional (3D) reproducibility of the isocenter positi on and of patient positioning with the use of bite block immobilizatio n by means of a simple verification procedure for a complex beam arran gement applied for ENT-tumors. Materials and methods: We analyzed the positioning data of 29 consecutive patients treated for ENT-tumors at the Department of Radiotherapy and Oncology of the University of Wurzb urg. A total of 136 treatment sessions were analyzed. Patients were po sitioned and immobilized using an individualized bite block system and a head and neck support. A complex beam arrangement was applied combi ning two offset rotational and two oblique wedge fields on a 5 MV line ar accelerator. Orthogonal verification films were taken once weekly. Four to six film pairs per patient were obtained (during 4-6 weeks) wi th a mean number of 4.7 film pairs per patient. These were compared to the corresponding orthogonal simulator films taken during primary sim ulation. Deviations of the verified isocenter from the isocenter on th e simulator film were measured and analyzed in three dimensions in ter ms of overall, systematic and random categories. A 3D-deviation vector was calculated from these 3D data as well as a 2D-deviation vector (f or comparison with literature data) from the lateral verification film s. Results: The overall setup deviation showed standard deviations (SD ) of 2.5, 2.7 and 3.1 mm along the cranio-caudal, anterior-posterior a nd medio-lateral axes, respectively. The random component ranged from SD 1.9 to 2.1 mm and the systematic component ranged from SD 1.8 to 2. 2 mm. The mean length of the 3D-vector was 3.1 mm for the systematic a s well as the random component. Ninety percent of 3D systematic and ra ndom deviations were less than 5 mm. The mean length of the 2D-vector was 2.4 mm for the random component and 2.2 mm for the systematic comp onent. Ninety percent of 2D-random and systematic variations were less than 4 mm. Conclusions: The presented individualized bite block immob ilization device provides an accurate and reproducible patient positio ning for SD-conformal radiation therapy in the head and neck. Random a nd systematic deviations in each of the three directions are in the ra nge of +/- mm (2 SD, comprising 95% of the deviations) and are within the range or even less than deviations described far most thermoplasti c Or PVC-mask fixation devices. These deviations should be taken into account during definition of planning target volume in head and neck t umors. (C) 1997 Elsevier Science Ireland Ltd.