Variability of Target and Normal Structure Delineation for Breast Cancer Radiotherapy: An RTOG Multi-Institutional and Multiobserver Study

Abstract

Purpose To quantify the multi-institutional and multi-observer variability of target and organ-at-risk (OAR) delineation for breast-cancer radiotherapy (RT), and its dosimetric impacts, as the first step of a RTOG effort to establish a breast cancer atlas. Methods and Materials Nine radiation oncologists specializing in breast RT from eight institutions independently delineated targets (e.g., lumpectomy cavity, boost planning target volume, breast, supraclavicular, axillary and internal mammary nodes, and chest wall) and OARs (e.g., heart, lung) on the same CT images of three representative patients with breast cancer. Inter-observer differences in structure delineation were quantified with regard to volume, distance between centers of mass, percent overlap, and average surface distance. The mean, median and standard deviation for these quantities were calculated for all possible combinations. To asses the impact of these variations on treatment planning, representative dosimetric plans based on observer-specific contours were generated. Results The variability in contouring the targets and OARs between the institutions/observers was substantial. The structure overlaps were as low as 10% and the volume variations had standard deviations up to 60%. The large variability was related both to differences in opinion regarding target and OAR boundaries as well as approach to incorporation of setup uncertainty and dosimetric limitations in target delineation. These inter-observer differences result in substantial variations in dosimetric planning for breast RT. Conclusions The differences in target and OAR delineation for breast irradiation between institutions/observers appear to be clinically and dosimetrically significant. A systematic consensus is highly desirable, particularly in the era of IMRT/IGRT.