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. 2012 Sep;85(1017):e654-60.
doi: 10.1259/bjr/76424694.

Interobserver delineation variation in lung tumour stereotactic body radiotherapy

G F Persson  1 , D E Nygaard, C Hollensen, P Munck af Rosenschöld, L S Mouritsen, A K Due, A K Berthelsen, J Nyman, E Markova, A P Roed, H Roed, S Korreman, L Specht
Affiliations

Interobserver delineation variation in lung tumour stereotactic body radiotherapy

G F Persson et al. Br J Radiol. 2012 Sep.

Abstract

Objectives: In radiotherapy, delineation uncertainties are important as they contribute to systematic errors and can lead to geographical miss of the target. For margin computation, standard deviations (SDs) of all uncertainties must be included as SDs. The aim of this study was to quantify the interobserver delineation variation for stereotactic body radiotherapy (SBRT) of peripheral lung tumours using a cross-sectional study design.

Methods: 22 consecutive patients with 26 tumours were included. Positron emission tomography/CT scans were acquired for planning of SBRT. Three oncologists and three radiologists independently delineated the gross tumour volume. The interobserver variation was calculated as a mean of multiple SDs of distances to a reference contour, and calculated for the transversal plane (SD(trans)) and craniocaudal (CC) direction (SD(cc)) separately. Concordance indexes and volume deviations were also calculated.

Results: Median tumour volume was 13.0 cm(3), ranging from 0.3 to 60.4 cm(3). The mean SD(trans) was 0.15 cm (SD 0.08 cm) and the overall mean SD(cc) was 0.26 cm (SD 0.15 cm). Tumours with pleural contact had a significantly larger SD(trans) than tumours surrounded by lung tissue.

Conclusions: The interobserver delineation variation was very small in this systematic cross-sectional analysis, although significantly larger in the CC direction than in the transversal plane, stressing that anisotropic margins should be applied. This study is the first to make a systematic cross-sectional analysis of delineation variation for peripheral lung tumours referred for SBRT, establishing the evidence that interobserver variation is very small for these tumours.

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Figures

Figure 1
Figure 1
Schematic figure showing the contours delineated by the six observers on a single slice. Each colour represents an observer and the dots represent the centre of volume (CoV) of each contour. A reference point is calculated as the mean position of the CoVs (black dot). A mean contour is calculated (black line) as the mean distance from the reference point to the six contours in 360 equally spaced angles radiating from the reference point. A magnification is inserted in the right lower corner: the distances from each contour to the mean contour, in 360 equally spaced angles radiating from the reference point, were measured and a local standard deviation calculated.
Figure 2
Figure 2
(a) Transversal view with contours of the tumour with the largest interobserver delineation variation in the transversal plane. Standard deviation of distance to a reference contour calculated for the transversal plane (SDtrans) was 0.36 cm and tumour volume was 16.3 cm3. (b) Transversal views with contours of a tumour with a small interobserver delineation variation in the transversal plane. SDtrans was 0.07 cm and tumour volume was 1.5 cm3. Radiologists' contours are red, oncologists' contours are green and the rough positron emission tomography contour is blue.
Figure 3
Figure 3
(a) Sagittal views with contours of the tumour with the second largest interobserver delineation variation in the craniocaudal direction. Standard deviation of distance to a reference contour calculated for the craniocaudal direction (SDcc) was 0.62 cm and tumour volume was 19.7 cm3. (b) Sagittal views with contours of two tumours in the right lung (the patient had three tumours). The caudal tumour had a small interobserver delineation variation in the craniocaudal direction, SDcc being 0.11 cm. Standard deviation of distance to a reference contour calculated for the transversal plane (SDtrans) was 0.09 cm and tumour volume was 0.7 cm3. The cranial tumour in the right image had a SDcc of 0.23 cm (SDtrans was 0.09 cm and tumour volume was 2.1 cm3). Radiologists' contours are red, oncologists' contours are green and the rough positron emission tomography contour is blue.
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