Differences in target outline delineation from CT scans of brain tumours using different methods and different observers.

PURPOSE: To assess errors resulting from manual transfer of contour information for three-dimensional (3-D) target reconstruction, and to determine variations in target volume delineation of brain tumours by different radiation oncologists.
MATERIALS AND METHODS: Images of 18 patients with intracranial astrocytomas were used for retrospective treatment planning by five radiation oncologists. In this study, the target outline was delineated on sequential CT slices by an experienced radiation oncologist. Thereafter, the target outline was manually reconstructed by five radiation oncologists onto an A-P or lateral scout film. The same target outline was also reconstructed as a projection using the Beam's-eye view capability on a CT simulator unit. The two target outlines were compared by encompassing each shape with the smallest rectangle. The manually-reconstructed radiation field was termed 'Field manually established on X-ray film (F-X)', and the automatically-established field was termed 'Field established by CT simulator (F-CT)'. In a second part of this study, four radiation oncologists defined contours from contrast enhanced CT images of nine patients with intracranial astrocytomas. The CT images of these nine cases included five pre-operative cases and four post-operative cases. Both gross tumour volume (GTV) and clinical target volume (CTV) were outlined on sequential CT slices. The target outlines for the four radiation oncologists were compared by identifying the smallest rectangular field surrounding the projection of these contours. The field established by each radiation oncologist was termed 'Field of target volume (F-TV)', and the overlapping portion of the four F-TVs for each case was termed 'Overlapped field of the target volume (Fo-TV)'.
RESULTS: The average distance between the isocentres of F-X and F-CT was 0.6 +/- 0.4 cm (mean +/- SD). The average ratio of the area of F-X divided by the area of F-CT was 1.04 +/- 0.12. The area of F-X was wider than the area of F-CT for four of the five oncologists. The ratio of the area of F-TV divided by the area of Fo-TV was calculated. The average ratio was relatively greater for CTV (2.07 in pre-operative cases and 2.11 in post-operative cases) than for GTV (1.12 in pre-operative cases and 1.41 in post-operative cases). Among radiation oncologists, variations in the delineation of GTV were smaller than those of CTV.
CONCLUSIONS: When using an X-ray simulator in treatment planning, errors resulting from the manual transfer of CT contour information to planar radiographs must be considered. When computer techniques are used to project contours onto radiographs errors resulting from individual variations when performing the contouring must be considered.