Decades of research have shown that breast cancer is a disease with inter-tumour heterogeneity. The phenotypic expression of distinct clinically relevant biological differences result in very heterogeneous behavior within the one cancer type, for example luminal, triple-negative, HER2-positive breast cancer subtypes as defined by the tumour expression of ER/PR, HER2 and Ki67.

Traditionally, adjuvant systemic therapy was applied after breast cancer surgery. According to the St Gallen 2015 international expert consensus, the decision of systemic adjuvant therapies for early breast cancer was based on the pathological stage and the surrogate intrinsic phenotype as determined by the ER/PR, HER2 and Ki67 assessment.

Recently, preoperative systemic therapy i.e.(neo)adjuvant therapy, is increasingly an option. Part of the interest in systemic treatment before surgery can be related to the potential down-staging effect that allows less extensive primary breast cancer surgery. More importantly, recent data have demonstrated that pathologic response (especially if complete) after preoperative systemic therapy correlates with improved survival outcome, especially for triple-negative and HER2-positive breast cancer subtypes. The in-vivo assessment of sensitivity or resistance is thus an important clinical indicator for deciding the post-surgical cancer treatment and redefining the disease prognosis.

Nowadays, new molecular target drugs and regimens are being developed and based on the predicted sensitivity for specific breast cancer histological types. In this era of precision medicine, the identification of novel biological predictive markers that are unique to the individual patients and their tumours can direct the appropriate targeted therapy for achieving the optimal treatment efficacy.