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Accurate, consistent and reproducible grading by pathologists is of key‐importance for identification of individual patients with invasive breast cancer (IBC) that will or will not benefit from adjuvant systemic treatment. We studied the laboratory‐specific grading variation using nationwide real‐life data to create insight and awareness in grading variation. Synoptic pathology reports of all IBC resection‐specimens, obtained between 2013 and 2016, were retrieved from the nationwide Dutch Pathology Registry (PALGA). Absolute differences in laboratory‐proportions of Grades I–III were compared to the national reference. Multivariable logistic regression provided laboratory‐specific odds ratios (ORs) for high‐  vs . low‐grade IBC. 33,792 IBC pathology reports of 33,043 patients from 39 laboratories were included, of which 28.1% were reported as Grade I (range between laboratories 16.3–43.3%), 47.6% as Grade II (38.4–57.8%), and 24.3% as Grade III (15.5–34.3%). Based on national guidelines, the indication for adjuvant chemotherapy was dependent on histologic grade in 29.9% of patients. After case‐mix correction, 20 laboratories (51.3%) showed a significantly deviant OR. Significant grading differences were also observed among pathologists within laboratories. In this cohort of 33,043 breast cancer patients, we observed substantial inter‐ and intra‐laboratory variation in histologic grading. It can be anticipated that this has influenced outcome including exposure to unnecessary toxicity, since choice of adjuvant chemotherapy was dependent on grade in nearly a third of patients. Better standardization and training seems warranted.

Significant inter‐ and intra‐laboratory variation in grading of invasive breast cancer: A nationwide study of 33,043 patients in the Netherlands