Comparison of two immobilization techniques using portal film and digitally reconstructed radiographs for pediatric patients with brain tumors

Purpose: To compare the accuracy of two immobilization techniques for pedia tric brain tumor patients. Methods and Materials: We analyzed data from 128 treatments involving 22 pa tients. Patients were immobilized with either a relocatable head frame (12 patients) or a vacuum bag (10 patients). Orthogonal portal films were used as verification images. Errors in patient positioning were measured by comp aring verification images with digitally reconstructed radiographs generate d by a three-dimensional treatment-planning system. Results: With the head frame, systematic errors ranged from 1.4 mm to 2.1 m m; random errors, from 1.7 mm to 2.1 mm. With the vacuum bag, systematic er rors ranged from 2.1 mm to 2.5 mm; random errors, from 2.0 mm to 2.6 mm, Fo r the head frame, the mean length of the radial displacement was 4.4 mm; 90 % of the total three-dimensional deviation was less than 6.8 mm, The corres ponding values for the vacuum bag were 5.0 and 6.6 mm, respectively. Conclusions: The head frame and vacuum bag techniques limit the random and systematic errors in each of the three directions to within +/-5 mm, We hav e used these results to determine the margin used to create the planning ta rget volume for conformal radiation therapy. (C) 2000 Elsevier Science Inc.