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Abstract: Literature shows that HER2/neu positive breast cancer cells are more sensitive to radiation-induced apoptosis by targeting the epidermal growth factor receptor family tyrosine kinase. We selected 466 patients with pT1-2 HER2/neu positive tumors who received adjuvant trastuzumab for primary invasive breast cancer. Patients were divided into three groups [Quadrantectomy followed by conventional radiotherapy vs Quadrantectomy followed by Intra-operative radiotherapy with electrons vs Mastectomy without radiotherapy]. After a median follow-up of 52 months, the 5-year cumulative incidence of locoregional recurrence (LRR) was 1.9%, 11.5% and 5.0% respectively (p < 0.01). At the multivariate analysis, extensive perivascular invasion, Luminal B HER2/Progesterone Receptor (PgR) negative status and Quadrantectomy followed by Intra-operative radiotherapy with electrons have significantly increased the risk of LRR. Our results suggest that HER2/neu positive breast cancer might have better outcomes when treated simultaneously with external radiotherapy and trastuzumab. Moreover, we underline the importance of PgR and further new stratification of risk among luminal subtypes. [Copyright &y& Elsevier]

Cyclin-Dependent Kinase 4/6 inhibitors (CDK4/6is) in combination with Endocrine Therapy (ET) represent the standard frontline therapy for patients with Hormone Receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic Breast Cancer (mBC). Clinical activity and efficacy of CDK4/6is-based therapies have been proven both in the endocrine sensitive and resistant settings. Therapy resistance eventually underpins clinical progression to any CDK4/6is-based therapies, yet there is a lack of validated molecular biomarkers predictive of either intrinsic or acquired resistance to CDK4/6is in clinical practice. As the "post-CDK4/6is" landscape for the management of HR-positive/HER2-negative mBC is rapidly evolving with the introduction of novel therapies, there is an urgent need for the definition of clinically relevant molecular biomarkers of intrinsic/acquired resistance mechanisms to CDK4/6is. This narrative review outlines the role of currently approved CDK4/6is-based therapies, describes the most relevant molecular biomarkers of CDK4/6is-resistance, and ultimately provides a perspective on the clinical and research scenario. [Display omitted] • Cyclin-Dependent Kinase 4/6 inhibitors (CDK4/6is)-based combinations represent the frontline therapy in Hormone Receptor positive, Human Epidermal Growth Factor 2 negative (HR+/HER2-) metastatic Breast Cancer (mBC). • Patients with HR+ /HER2- mBC eventually develop resistance to CDK4/6is-based combinations, with approximately 15–30 % displaying rapid progression. • Molecular biomarkers of intrinsic and acquired resistance to CDK4/6is-therapy are currently lacking in clinical practice and are eagerly awaited. [ABSTRACT FROM AUTHOR]