Abstract Aim. Ductal carcinoma in situ (DCIS) is considered to be a non-obligate pre-invasive precursor of invasive ductal carcinoma. We previously showed that DCIS with a predominantly myxoid periductal stromal architecture is associated with an increased risk of both overall and invasive recurrence. The aim of this study is to determine a cut-off for the assessment of myxoid stroma and stromal inflammation in DCIS, based on inter-observer variability. Here, preliminary results of the histopathological analysis of 285 DCIS patients are presented, in which the consistency of assessment of stromal features is compared to the reproducibility of other morphological characteristics. Methods. Hematoxylin/eosin stained tissue sections of 285 DCIS lesions were retrieved from the archives of the Department of Pathology of Leuven University Hospitals, Leuven, Belgium. The following characteristics were independently scored by two pathologists: nuclear grade, intraductal calcifications, extensive comedonecrosis, DCIS architecture, stromal architecture and stromal inflammation. Nuclear grade was scored as low, intermediate or high grade. Intraductal calcifications were scored as absent or present. Extensive comedonecrosis was defined as eosinophilic necrotic debris in >50% of ductal lumina. DCIS architecture was categorized as non-solid or solid, with a cut-off at 50% of ducts presenting with solid growth. Myxoid stroma was defined as loosely arranged collagen fibers interspersed with an amorphous, slightly basophilic substance. Stromal architecture was classified into 4 categories (0%, 1-33%, 33-66% or >66% myxoid stroma). By applying identical cut-offs, stromal inflammation was subdivided into absent, mild, moderate or extensive periductal inflammation. All features were dichotomized, using different cut-offs. Kappa values were determined to assess inter-observer variability. Results. Nuclear grade was dichotomized as low grade versus intermediate/high grade (κ 0,500), and as grade low/intermediate versus high grade (κ 0,507). The kappa value for scoring myxoid stromal architecture was highest by dichotomization with a cut-off at 33% (κ 0,566), compared to κ 0,454 and κ 0,501 when using 1% and 66% as a cut-off, respectively. A similar analysis for stromal inflammation revealed that the highest kappa value was obtained by dichotomization as 'absent to mild' versus 'moderate to extensive' inflammation (κ 0,724). Dichotomization with cut-offs of 1% and 66% resulted in lower kappa values of κ 0,564 and κ 0,670, respectively. Scores for extensive comedonecrosis showed substantial agreement (κ 0,604). Scores for solid versus non-solid DCIS architecture (κ 0,507), and presence or absence of calcifications (κ 0,664) showed moderate and substantial agreement, respectively. Conclusions. Adequate prognostic markers require robustness of assessment, i.e. low inter-observer variability and thus high reproducibility. The dichotomous assessment of stromal features in DCIS resulted in similar or even higher kappa values compared to the dichotomous scoring of other histopathological characteristics. This study demonstrates the robustness of dichotomous assessment of both stromal architecture and stromal inflammation. Citation Format: Van Bockstal M, Lambein K, Smeets A, Nevelsteen I, Neven P, Christiaens M-R, Libbrecht L, Floris G. Assessment of stromal characteristics in ductal carcinoma in situ of the breast: An inter-observer variability study [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-03-14.