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Objective: To assess the contribution of unknown institutional factors (contextual effects) in the de-implementation of cALND in women with breast cancer. Summary of Background Data: Women included in the National Cancer Database with invasive breast carcinoma from 2012 to 2016 that underwent upfront lumpectomy and were found to have a positive sentinel node. Methods: A multivariable mixed effects logistic regression model with a random intercept for site was used to determine the effect of patient, tumor, and institutional variables on the risk of cALND. Reference effect measures were used to describe and compare the contribution of contextual effects to the variation in cALND use to that of measured variables. Results: By 2016, cALND was still performed in at least 50% of the patients in a quarter of the institutions. Black race, younger women and those with larger or hormone negative tumors were more likely to undergo cALND. However, the width of the 90% reference effect measures range for the contextual effects exceeded that of the measured site, tumor, time, and patient demographics, suggesting institutional contextual effects were the major drivers of cALND de-implementation. For instance, a woman at an institution with low-risk of performing cALND would have 74% reduced odds of having a cALND than if she was treated at a median-risk institution, while a patient at a high-risk institution had 3.91 times the odds. Conclusion: Compared to known patient, tumor, and institutional factors, contextual effects had a higher contribution to the variation in cALND use. [ABSTRACT FROM AUTHOR]