Breast-conserving treatment (BCT) constitutes the predominant approach to local treatment in early stage invasive breast cancer. This modality of treatment includes breast-conserving surgery (BCS), radiotherapy, and systemic treatment. These three pillars of BCT have been intensively studied during the past few decades to determine their role and possible variations in their application.

This panel discusses the state of the art and the current developments in radiotherapy for patients with early stage invasive breast cancer treated with BCT. Different radiotherapy options are systematically presented according to particular clinical scenarios and available scientific data.

Whole breast irradiation (WBI) represents the radiation 'gold standard' after BCS in breast cancer stages I–II. Several randomized trials, enrolling thousands of patients and with long-term follow up, have shown a clear improvement in local control when WBI is used after BCS. A recent meta-analysis has confirmed a threefold increase in local control rates. In addition, an 8.6% decrease in the risk for death was demonstrated [1].

Adjuvant chemotherapy and hormone therapy also contribute to increase local control rates in this group of patients [2]. Substantial efforts have been made to identify a low-risk subgroup of patients who do not benefit from radiotherapy after BCS. However, this subgroup has not been yet identified because even low-risk patients (T <2 cm, margin negative, EIC negative, age >70 years) do benefit from adjuvant WBI [3]. Nevertheless, several well known clinical and pathological factors define a profile of lower risk for local relapse in which more conservative radiotherapy modalities are being explored.

In this context accelerated partial breast irradiation (APBI) appears to be a promising alternative to WBI in selected patients, with possible similar efficacy, a considerable reduction in the treatment length with a resultant improved quality of life, and potential decreased toxicity. Different APBI techniques can be used, such as intraoperative electrons, catheter-based interstitial brachytherapy, MammoSite Balloon brachytherapy, or external-beam partial irradiation. Encouraging results with adequate recruitment and medium term follow up have been published in terms of local control and tolerability, the majority of them with the use of catheter-based interstitial brachytherapy [4]. However, some concerns remain, particularly regarding potential late adverse effects and potential differences among techniques. Patient selection, expertise, and high quality technology and assurance are key elements to the success of this emerging approach. Current multicentric randomized trials are ongoing and hopefully will help to define the ideal criteria for patient selection, the most satisfactory treatment modality, and the exact role of APBI in terms of outcome and toxicity.

Although WBI remains the radiation standard of care in early-stage invasive breast cancer after BCS, APBI emerges as promising approach for treating selected patients.