Breast cancer is one of the most common cancers in Western countries and the first cause of death among women in France. Studies have reported that the compliance of the treatment with breast cancer clinical practice guidelines (CPGs) is accompanied with a significant improvement of recurrence-free survival and overall survival rates. However, compliance of multidisciplinary tumour board decisions with CPGs remains non-satisfactory. The European project DESIREE aims at building a software package to support the guideline-based management of breast cancer patients. The aim is to select multiple relevant contemporary CPGs published on the management of breast cancer and to concurrently apply them to benefit of the complementarity of the recommendations issued, and leverage the guideline-based decision support. We used a clinical case to compare NCCN and ESMO CPGs in terms of concordance, complementarity, and conflicts. Out of the 757 decision rules extracted from both CPGs, 64 rules were triggered (32 from NCCN, and 32 from ESMO) by the clinical case. Ten rules were concordant, 52 complementary, and two conflicting. Complementarity relies on the different levels of granularity of the IF-parts of the rules that lead to different specific THEN-parts. Another type of complementarity comes from the provision of specific care apart from the breast cancer therapeutic management. The few inter- and intra-CPGs conflicts show the limits of CPGs in particular situations still debated by medical research.