Interobserver Variation in Tumor Delineation of Liver Metastases using Magnetic Resonance Imaging.

A cohort of 20 liver metastases treated on a MR-Linac were consecutively selected per primary tumor (colorectal (6), breast (6) and lung (8)). Planning MRI scans (T1 weighted with IV-contrast) were collected and anonymized. GTV delineation guidelines and case-specific information were provided to 8 radiation oncologists from 8 institutions. All cases were quantitatively reviewed and delineations with major violations of the guidelines were marked as outliers (such as contours including a vein or excluding obvious parts of a tumor). IOV was quantified by comparing individual delineations with the median contour and calculating the standard deviation (SD) and 95^th percentile Hausdorff distance (HD95). Analyses were conducted on all delineations and on a subgroup excluding outliers per case to distinguish between accuracy of delineation and individual guideline interpretation. Sphericity was calculated from the volume and surface area of all delineations. Correlation between SD and sphericity was determined using Spearman's r_s. Differences in SD between primary tumors were analyzed using the Kruskal-Wallis test.

The median volume of all metastases was 6.5 cc (IQR 3.7 - 29.8 cc) and sphericity was 0.84 (IQR 0.80 - 0.87) based on all contours (Table 1). The SD was 1.6 mm and the median HD95 was 2.7 mm. In the subgroup analysis, one (in 15 cases) or two (in 5 cases) delineations were excluded. Subgroup analyses showed a SD of 1.1 mm and median HD 95 of 2.6mm. The Kruskal-Wallis test showed no difference (p = 0.59) in SD between primary tumors. There was a significant negative correlation between IOV (SD) and sphericity (r_s = -0.70; p = 0.008). Table 1: Overview of SD (mm), HD95 (mm) and volume (cc) of all observers and subgroup per primary tumor and in total CONCLUSION: MRI based GTV delineation variation of liver metastases is 1.1 - 1.6 mm, which is smaller than anticipated, although significant outliers (HD95 up to 3.9 mm) were present. Use of common delineation guidelines will ensure consistency in contouring and have the potential to decrease treatment margins when taking IOV into account.

Magnetic Resonance (MR-) guided stereotactic body radiotherapy enables accurate treatment of liver metastases. Despite the accuracy of treatment delivery, one of the main uncertainties is the variation in tumor delineation, which requires additional treatment margins. The aim of this study was to quantify the interobserver variation (IOV) in MRI based delineation of the gross tumor volume (GTV) of liver metastases.