Variability in prostate and seminal vesicle delineations defined on magnetic resonance images, a multi-observer, -center and -sequence study

Abstract

Background: The use of magnetic resonance (MR) imaging as a part of preparation for radiotherapy is increasing. For delineation of the prostate several publications have shown decreased delineation variability using MR compared to computed tomography (CT). The purpose of the present work was to investigate the intra- and inter-physician delineation variability for prostate and seminal vesicles, and to investigate the influence of different MR sequence settings used clinically at the five centers participating in the study.

Methods: MR series from five centers, each providing five patients, were used. Two physicians from each center delineated the prostate and the seminal vesicles on each of the 25 image sets. The variability between the delineations was analyzed with respect to overall, intra- and inter-physician variability, and dependence between variability and origin of the MR images, i.e. the MR sequence used to acquire the data.

Results: The intra-physician variability in different directions was between 1.3 - 1.9 mm and 3 - 4 mm for the prostate and seminal vesicles respectively (1 std). The inter-physician variability for different directions were between 0.7 - 1.7 mm and approximately equal for the prostate and seminal vesicles. Large differences in variability were observed for individual patients, and also for individual imaging sequences used at the different centers. There was however no indication of decreased variability with higher field strength.

Conclusion: The overall delineation variability is larger for the seminal vesicles compared to the prostate, due to a larger intra-physician variability. The imaging sequence appears to have a large influence on the variability, even for different variants of the T2-weighted spin-echo based sequences, which were used by all centers in the study.

Figure 1

The radial distances from the center of mass to each delineation is a function of the angles θ and ∅. The distance in different directions for an individual delineation was calculated as the average distance over a small solid angle in each direction, (left (l), left-posterior (lp), posterior (p), right-posterior (rp), right (r), anterior (a), superior (s), inferior (i)). The area A, corresponding to the right-posterior direction, illustrates the relation between the 3D and 2D representation. The example in the figure is based on one delineation of one prostate.

Figure 2

Illustration of the separation of the overall variability into inter- and intra-physician components. The red dots represent observations by an individual physician for the 25 patients. The red line indicate the average observation for the physician.

Figure 3

The standard deviation,$\mathbf{S}{\left[{\mathit{x}}_{\mathbf{p},\mathbf{q}}^{\mathbf{volume}}\right]}_{\mathbf{q}},$for the volume of the delineations for all 25 patients (prostate and vesicles), as function of the average volume,${\left[{\mathit{x}}_{\mathbf{p},\mathbf{q}}^{\mathbf{volume}}\right]}_{\mathbf{q}}$. Each dot represents a patient. The imaging center is indicated by the dot shape.

Figure 4

Examples of the image quality of the images from the different centers (prostate and seminal vesicles).

Figure 5

Examples of the delineations (prostate and seminal vesicles).