ESTRO 2021 Abstract Book

S936

ESTRO 2021

Conclusion Substantial dosimetric improvements can be achieved with DIBH technique for left-sided breast cancer patients, reducing the dose to the LAD artery and heart, particularly to the subvolumes closer to the chest wall, while preserving at the same time the dose to the target. PO-1126 Cardiotoxic effects of tangential breast irradiation? B. Rebecca 1 , T. Huebner 2 , U. Schaefer 1 1 Klinikum Lippe GmbH, Radiation oncology, Lemgo, Germany; 2 Klinikum Lippe GmbH, Radiology, Lemgo, Germany Purpose/Objective(s): To assess the potential risk of cardiovascular events (CVE) after postlumpectomy or postmastectomy irradiation restricted to tangential fields. Materials/Methods: We estimated the CVE-free survival in 1033 patients with breast cancer (BC) treated with breast or chest wall tangential fields between 2003 and 2009. 3D - Linac -based radiotherapy was carried out with 6 MeV or mixed 6/15 MeV. Total dose was 50 Gy, with or without a boost. No breath hold technique was used. Dose constraint (Dmean) for the left ventricle and left anterior descending coronary artery was 5 Gy and 15 Gy. CVE-free survival was estimated using the Kaplan-Meier-method (KM), whereby any new CVE's after radiotherapy were documented. Results was separated between left and right site irradiation, differences were tested of significance using the Log rank test. Results: Mean follow-up was 77 (3 - 211) months. The incidence of CVE overall was 16.0%: 6.7% in patients with right-sided BC, compared with 9.3% in left-sided cases. KM - estimation of CVE-free survival (mean) with left-sided vs. right-sided BC was 171 months vs 181 months; p Log rank = 0.26 (not significant). Conclusion: Patients irradiated for left-sided BC with tangential fields have no statistical significant higher incidence of CVE compared with those with right-sided cancer. However, on left side treatment, there is a slight nonsignificant increase in CVE's, that should be investigated further. PO-1127 Omission of axillary irradiation in breast cancer patients with ypN0 after neoadjuvant chemotherapy S. Sarandão 1 , F. Fernandes 1 , S. Costa 1 , J. Rodrigues 2,3 , I. Azevedo 1 , H. Pereira 1 1 Instituto Português de Oncologia do Porto, External Radiotherapy, Porto, Portugal; 2 Instituto Português de Oncologia do Porto, Cancer Epidemiology Group, IPO Porto Research Center, Porto, Portugal; 3 Universidade do Minho, Centre of Mathematics, Braga, Portugal Purpose or Objective Neoadjuvant chemotherapy (NAC) induces significant changes in the pathologic extension of disease. The potential of down-staging challenges the standard indications of surgery and adjuvant radiotherapy (RT). Considering the increment of pathological complete response with systemic therapy, there is a considerable lack of evidence in how to safely de-escalate locoregional treatment, namely the omission of axillary nodal irradiation (AI). Therefore, we assessed the impact of omitting AI in breast cancer patients with clinically node-positive (cN+) at diagnosis that had a pathological complete nodal response (pCRn) post-NAC. Materials and Methods We retrospectively collected data from patients with breast cancer treated at our institution, between January 2014 and December 2016. The selection criteria were biopsy proven node metastasis at presentation that had a pCRn post-NAC. The patients were divided in two groups, the ones that received AI (control group) and the group with omission of AI. All patients received whole breast or chest wall irradiation, according to surgery performed. The supraclavicular lymph nodes were irradiated in the two groups. The internal mammary was not systematically irradiated, depending on the tumor's location. Overall survival (OS) was estimated by the Kaplan-Meier method. The log-rank test was performed to compare survival rates between groups. Results Among the 68 eligible patients with ypN0, 32 patients (47%) received AI, and in 36 patients (53%) AI was omitted. It was observed statistical difference between groups regarding cN status (p=0.009) and the irradiated nodal areas (p=0.007). Majority of patients (77.8%) that omitted AI had cN1 at diagnosis. On the other hand, 56.3% of the control group were, at least, cN2. In the no-AI group, the irradiation of internal