Large Interobserver Variation in the International MR-LINAC Oropharyngeal Carcinoma Delineation Study

Abstract

Generally, a (contrast-enhanced) CT is used for delineation of oropharyngeal carcinomas (OPC’s). MRI however, has a superior soft tissue contrast. Adding MR images to the planning CT, might result in a reduced interobserver variation. MR-guided radiotherapy is in its infancy but will be the standard in the near future. Therefore, we want to determine the variation in target delineation for oropharyngeal cancer (OPC) by international experts using MR as a single delineation modality in order to develop guidelines for delineating OPC’s on MR images Twentyfour radiation oncologists from 7 centers affiliated with the MR-linac Consortium were asked to delineate the gross tumor volume (GTV) and the clinical target volume (CTV) of the primary tumor for 4 OPC cases. Observers were given a brief clinical history, a physical examination report and the T1-, T1- plus Gadolinium and T2- weighted MR images . Target contours were delineated according to experts’ institutional guidelines. The absolute and encompassing volumes, the generalized conformity index and median surface distances were then calculated from the voxel count maps of each case. Furthermore, the data were analyzed with the Simultaneous Truth and Performance Level Estimation (STAPLE) algorithm. The median years of experience for the participating radiation oncologists was 5 years (range 0-15 years). The mean absolute volumes of the GTVs of case 1, case 2, case 3 and case 4 were 6.5 ml (range: 3.9-9.9 ml), 8.6 ml (range 1.9-22.1 ml), 32.6 ml (range: 17.3-64.4 ml) and 20.3 ml (range 7.8-33.6 ml), respectively. The conformity indices for the GTV of the primary tumor were 0.6, 0.38, 0.5 and 0.59 for case 1, case 2, case 3 and case 4, respectively. The mean absolute volumes of the CTVs of case 1, case 2, case 3 and case 4 were 37.0 ml (range: 9.9 - 76.6 ml), 41.4 ml (range 19.7 - 108.5 ml), 113.8 ml (range: 48.5 - 254.7 ml) and 77.4 ml (range 22.7 - 140.6 ml), respectively. The conformity indices for the CTV of the primary tumor were 0.52, 0.46, 0.49 and 0.56 for case 1, case 2, case 3 and case 4 respectively. The STAPLE algorithm largely covers the median GTV voxel density. The interobserver variation of target volumes delineation of OPC patient based on MR is large. For the GTV, volume differences up to a factor of 10 were observed. For the CTV, differences up to of factor of 8 were observed.This demonstrates the need for international MR delineation guidelines for oropharyngeal cancer patients treated using MR-guided radiotherapy.