SPINAL-CORD LOCALIZATION IN THE TREATMENT OF LUNG-CANCER - USE OF RADIOGRAPHIC LANDMARKS

Purpose: In the treatment of thoracic malignancies with radiotherapy, the critical dose-limiting structure is the spinal cord, Oblique field s typically are designed to exclude the spinal cord, and by convention , the field edge that shields the spinal cord is placed at the anterio r border of the vertebral pedicles. Thus, the purpose of our study was to estimate the distance between the field edge and spinal cord in ob lique fields that were designed by using the vertebral pedicle as a ra diographic landmark. Methods and Materials: The spinal cord of a cadav er was wrapped in wire, and oblique fields were simulated at 15 degree s intervals, The distance from the spinal cord to a field edge placed at the anterior border of the pedicle was measured, In the second inve stigation, a three-dimensional treatment planning system was used to s imulate hypothetical fields using actual patient data from computed to mography (n = 10), and measurements identical to those in the anatomic al model were made (n = 1,100), Results: The results of the anatomical and computed tomographic models were in close agreement (mean differe nce, 0.6 mm). The computed tomographic model predicted a mean field ed ge to spinal cord distance of 8.7 mm (95% confidence interval, 5.6-11. 8 mm) for 30 degrees/150 degrees oblique fields and 8.0 mm (95% confid ence interval, 4.7-11.7 mm) for 45 degrees/135 degrees oblique fields, This distance was greatest at levels T-1, T-2, and T-11 (8 to 20% gre ater), Conclusions: The mean distance from a field edge placed at the anterior border of a vertebral pedicle to the spinal cord for commonly used oblique angles constitutes a sufficient margin to account for ex pected differences in daily positional variations and mechanical uncer tainties. (C) 1998 Elsevier Science Inc.