WHAT MARGINS SHOULD BE ADDED TO THE CLINICAL TARGET VOLUME IN RADIOTHERAPY TREATMENT PLANNING FOR LUNG-CANCER

Background: The planning target volume in radiotherapy treatment plann ing takes into account both movements of the clinical target volume (C TV) and set-up deviations. Materials and methods: A group of patients who received radiotherapy for lung cancer were studied. In order to me asure the CTV movements due to respiration and other internal organ mo tions, fluoroscopy was performed for 20 patients. To study the accurac y and reproducibility of patient and beam set-up, 553 electronic porta l images from 20 patients were evaluated. Discrepancies between planne d and actual field positions were measured and the systematic and rand om errors were identified. The combined effect of these geometrical va riations was evaluated. Results: The average CTV movement with quiet r espiration was about 2.4 mm in the medio-lateral and dorso-ventral dir ections. Movement in the cranio-caudal direction was on average 3.9 mm with a range of 0 - 12 mm. The systematic set-up errors were on avera ge 2.0 mm in the transversal plane and 3.0 mm in the cranio-caudal dir ection. The random errors can be described by their standard deviation s of 3.2 and 2.6 mm. In this study, the combined effect of the two par ameters (CTV movement and set-up deviations) varied between 7.5 and 10 .3 mm in different anatomical directions. Conclusions: In our daily cl inical routine, we use a margin of 11 mm in the transversal plane and 15 mm cranially and caudally, also taking into account other unquantif ied variations and uncertainties. (C) 1998 Elsevier Science Ireland Lt d. All rights reserved.