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.

Initial evaluation of automated treatment planning software

“…However, even with these techniques, and factoring out variations in contouring, 1 , 2 , 3 , 4 the quality of treatment plans can vary greatly. Nelms et al (5) reported a study where different institutions were asked to produce a plan based on the same downloadable CT dataset with presegmented targets and normal structures.…”

Inter-observer variation in delineating the coronary arteries as organs at risk

“…Consistency in delineation is crucial in further development of NTCP models for coronary arteries and in the clinical practice to optimize the RT plan for each patient. Several studies have documented significant inter-observer variation in delineation of target volumes and OARs [16][17][18]. One previously published study on reproducibility of delineating coronary arteries showed a substantial inter-observer variation in estimated dose [19].…”

Comparison of Incidental Radiation Dose to Axilla and Internal Mammary Nodal Area by Conventional, 3DCRT, and IMRT Technique in Carcinoma of Breast

“…The coverage increased to 86%, 71% and 73% with high tangential fields. 51% of level I, 26% of level II and 15% of level III received 95% (V95%) of the dose and this was increased to 79%, 51%, and 49% for high tangents (20). The average dose delivered to level I, II, III by IMRT technique was 80.7%, 69%, 52.3% and by conventional technique it was 85.8%, 67% and 41.8%.…”

“…That can have direct correlation, with standard tangents, the axilla receives a significant incidental dose but still adequate for clinical control of disease is unknown [18,19]. Substantial variability was observed in contouring the targets and organ at risk between observers and the structure overlaps were as low as 10% and volume variation had standard variation up to 60% [20]. The average volume of axillary level I, II, III as observed by Julia in 35 patients was 85 cc, 17 cc, 5 cc whereas the mean volume of the axillary level I was 28.9 cc (range 14.5 to 58.6 cc) in a study by Orecchia et al [21] in 15 patients.…”