OncoDoc2 is a guideline-based clinical decision support system (CDSS) applied to the management of breast cancer patients. OncoDoc2 has been routinely used during multidisciplinary staff meetings at the Tenon Hospital (Paris, France) for nearly 3 years. Despite the use of the CDSS that reminds physicians of the recommended treatments, the compliance rate of decisions is not 100%. We have used pattern mining techniques in order to elicit patient clinical profiles associated with non-compliance. We quantified each extracted pattern by three measures (support, growth rate, and unexpected rate) and we introduced a score to prune relevant emerging patterns. Non-compliance has concerned elderly patients in pre-surgery decisions, patients with micro invasive tumor in re-excision decisions, and patients HR+ and Her2+ in adjuvant decisions. In all cases, physician non-compliance with guidelines occurs when scientific evidence is lacking.