Interobserver variations in gross tumor volume delineation of brain tumorson computed tomography and impact of magnetic resonance imaging

Purpose: (1) To assess the interobserver variability of brain tumor delinea tion on computed tomography (CT). (2) To assess the impact of the addition of magnetic resonance imaging (MRI) information. Methods: Nine physicians were asked to delineate the gross tumor volume (GT V) of five patients with supratentorial inoperable brain tumors on CT scans and 2 weeks (or more) later on MRIs. The delineations were performed on a computer screen. During delineation on MRI, the registered CT images (witho ut delineation) were displayed on the screen (MRI + CT). Results: A high interobserver variability in GTV delineation on CT is found : the ratio of the largest to the smallest defined volumes varies for the f ive patients by factors of resp. 2.8, 1.8, 1.8, 1.9 and 1.7. The interobser ver variability is as large on MRI + CT as on CT alone (ratio largest/small est volume: 2.4, 1.7, 1.9, 2.7 and 1.5). Volumes delineated on MRI + CT (me an: 69.6 cm(2)) are larger than on CT alone (mean: 59.5 cm(3)). Residual vo lumes (volume delineated on one image modality but not on the other) are >0 for CT alone and for MRI + CT. Conclusions: A large interobserver variability in GTV delineation of brain tumors is demonstrated. The addition of MRI to CT does not reduce interobse rver variability. GTVs delineated on MRI + CT are larger than on CT alone, but some volumes are delineated on CT and not on MRI. Therefore, a combinat ion of the two image modalities is recommended for brain tumor delineation for treatment planning. (C) 2001 Elsevier Science Ireland Ltd. All rights r eserved.