ESTRO 2025 - Abstract Book

S118

Invited Speaker

ESTRO 2025

occurred more (x1.9) and earlier without (31 months) vs. with (43.6 months) RT group. No significant differences in distant recurrence or survival were seen. In the EORTC 22922/10925 trial, 1778 of the 4004 patients with pathologically node negative disease received regional nodal RT or not. Independent from the nodal status, a significant reduction of breast cancer-related mortality and of any recurrence by RT was seen, without improvement of overall survival. The benefit from RT might be more expressed if the internal mammary nodes are irradiated and if more effective systemic therapy is given. In the EBCTCG meta-analysis of regional nodal RT trials, including 2188 node negative patients, the proportional reduction of recurrences and mortality was independent of risk factors, while the absolute improvements were larger for patients at higher risk for recurrences and mortality: the absolute 15-year breast cancer mortality reductions were 1–2% for N0, 2–3% for pN1 and 4–5% for pN2-3 disease stages. The only influencing factor was the side of the tumour within the breast, with a larger benefit for centrally and medially located tumours. No increased non-breast-cancer mortality was observed. In summary, elective nodal irradiation improves disease-free survival for patients without involved lymph nodes in the presence of risk factors and/or in case of central/medial location. The absolute benefit depends on the absolute recurrence risks in combination with the effectiveness of systemic therapy. The side effects of RT delivered with contemporary techniques are very limited.

4845

Speaker Abstracts Regional radiotherapy after primary systemic therapy Liesbeth Boersma Department of Radiation Oncology (Maastro), GROW Research Institute for Oncology and Reproduction, Maastricht University Medical Centre+, Maastricht, Netherlands

Abstract:

In the early days, the indication for radiotherapy (RT) was based upon the post-operative stage and pathology. With the increasing application of primary systemic treatment (PST), it became challenging to determine the indications for post-operative RT. The Houston group clearly showed that when basing the indication for RT solely on the post chemotherapy and post-operative pathology, the 5 year loco-regional recurrence rate increased significantly, i. e. from 15% to 27% 1 . Therefore, it was explicitly stated that the pre-operative staging should be taken into account as well when deciding on RT. In addition, since the changed sequence of chemotherapy and the loco-regional treatment did not appear to influence overall outcome, it was thought that application of the different modalities in itself should not be changed. Consequently, an axillary lymph node dissection (ALND) followed by locoregional RT was usually applied in case of positive lymph nodes prior to PST, since the EBCTCG analyses had shown a breast cancer specific survival benefit in case of pN+ disease 2 . However, with the improved survival rates and reduced rates of locoregional recurrences, improved selection criteria for locoregional therapy has gained attention, i.e. de-escalation where possible in order to improve Quality of Life of survivors, and escalation when needed, to improve survival and/or loco-regional control. In this presentation, de-escalation studies will be reviewed for a variety of scenarios: 1. In case an ALND is performed after PST. 2. In case only a surgically axillary re-staging procedure (SARP) is performed, either by removal of the sentinel node, a marked node, or both. Ad 1) One of the first studies aimed to de-escalate RT was the RAPCHEM study; in this prospective cohort study, chestwall RT was omitted after mastectomy in case of ypN0 disease, and regional RT was omitted in case of ypN1 disease. Overall the 5-year loco-regional recurrence rates were < 2.3% in this study 3 , suggesting that the chosen de escalation strategy was safe. In patients with ypN0 disease, this finding was indeed confirmed in the NSABP B 51/RTOG1304 study, presented at SABCS 2023, where 45% of the patients underwent an ALND, and 55% of the patients only underwent SARP. In this trial patients were randomized between no loco-regional RT vs comprehensive