Inter-observer variability of organ contouring for preclinical studies with cone beam Computed Tomography imaging

Abstract

Background and purpose: In preclinical radiation studies, there is great interest in quantifying the radiation response of healthy tissues. Manual contouring has significant impact on the treatment-planning because of variation introduced by human interpretation. This results in inconsistencies when assessing normal tissue volumes. Evaluation of these discrepancies can provide a better understanding on the limitations of the current preclinical radiation workflow. In the present work, interobserver variability (IOV) in manual contouring of rodent normal tissues on cone-beam Computed Tomography, in head and thorax regions was evaluated.

Materials and methods: Two animal technicians performed manually (assisted) contouring of normal tissues located within the thorax and head regions of rodents, 20 cases per body site. Mean surface distance (MSD), displacement of center of mass (ΔCoM), DICE similarity coefficient (DSC) and the 95th percentile Hausdorff distance (HD₉₅) were calculated between the contours of the two observers to evaluate the IOV.

Results: For the thorax organs, right lung had the lowest IOV (ΔCoM: 0.08 ± 0.04 mm, DSC: 0.96 ± 0.01, MSD:0.07 ± 0.01 mm, HD₉₅:0.20 ± 0.03 mm) while spinal cord, the highest IOV (ΔCoM:0.5 ± 0.3 mm, DSC:0.81 ± 0.05, MSD:0.14 ± 0.03 mm, HD₉₅:0.8 ± 0.2 mm). Regarding head organs, right eye demonstrated the lowest IOV (ΔCoM:0.12 ± 0.08 mm, DSC: 0.93 ± 0.02, MSD: 0.15 ± 0.04 mm, HD₉₅: 0.29 ± 0.07 mm) while complete brain, the highest IOV (ΔCoM: 0.2 ± 0.1 mm, DSC: 0.94 ± 0.02, MSD: 0.3 ± 0.1 mm, HD₉₅: 0.5 ± 0.1 mm).

Conclusions: Our findings reveal small IOV, within the sub-mm range, for thorax and head normal tissues in rodents. The set of contours can serve as a basis for developing an automated delineation method for e.g., treatment planning.

Keywords: CT; Interobserver variability; Organ contouring; Preclinical.

Fig. 1

First row: mouse organs segmented in the thorax region depicted in 3D and 2D planes. Yellow: left lung; Orange: right lung; Green: spinal cord; Blue: heart; Beige: thorax bone. Second row: rat organs segmented in the head region depicted in 3D and 2D planes. Yellow: left brain hemisphere; Orange: right brain hemisphere; Green: left eye; Blue: right eye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Boxplots of evaluation metrics for the different organs in the thorax region of 20 mice. The whiskers depict the 25th and 75th percentile; the dashed line indicates the median value, the triangle represents the mean value, and the circles represent outliers.

Fig. 3

Boxplots of evaluation metrics for the different organs in the head region of 20 rats. The whiskers depict the 25th and 75th percentile; the dashed line indicates the median value, the triangle represents the mean value, and the circles represent outliers.

Fig. 4

Contoured organs by two annotators (blue: annotator #1; red: annotator #2) and region of IOV (yellow). Examples are shown for worst, intermediate and best cases per organ. First row: heart Second row: spinal cord; Third row: thorax bone; Forth row: complete lung. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Contoured organs by two annotators (blue: annotator #1; red: annotator #2) and region of IOV (yellow). Examples are shown for worst, intermediate and best cases per organ. First row: left hemisphere; Second row: complete brain; Third row: right eye.