ESTRO 2021 Abstract Book

S937

ESTRO 2021

mammary nodes was omitted in 86.1% of patients. All the patients underwent axillary dissection, except one that was submitted to sentinel lymph node biopsy. The median follow-up was 57.6 months. The OS and progression free survival (PFS) in the control group were 80.2% and 81.2%, respectively. In the group with omission of AI, OS and PFS were 91.7% and 90.5%. During follow-up, 9 patients (13.2%) had recurrence (locoregional or at distance), 6 on control group and 3 on group with AI omission. At the time of study, 91.7% of patients with AI omission were alive and all without evidence of disease, compared with 81.2% in the control group. Conclusion Our findings suggest that omission of AI can be appropriate for breast cancer patients with cN1 at diagnosis that achieve pCRn. However, we must point out that cN2 and cN3 patients have a worse prognosis, and therefore omission of AI should not be considered. Further randomized trials are needed to study how to personalize the radiotherapy axillary approach post-NAC. PO-1128 Oesophagus exposure in breast cancer radiotherapy: systematic review of oesophageal doses 2010-2020 F. Duane 1,2,5 , A. Kerr 3 , M. Aznar 4 , Z. Wang 3 , G. Ntentas 3,6 , S. Darby 3 , C. Taylor 3 1 Radiation Oncology, St Luke's Radiation Oncology Network, Dublin, Ireland; 2 School of Medicine, Trinity College Dublin, Dublin , Ireland; 3 Clinical Trials Service Unit, Nuffield Department of Population Health, University of Oxford, Oxford, United Kingdom; 4 Manchester Cancer Research Centre, Division of Cancer Sciences, Faculty of Biology, Medicine and Health, University of Manchester, Manchester, United Kingdom; 5 Trinity St James's Cancer Institute, St James's Hospital, Dublin, Ireland; 6 Department of Physics, Guy’s and St Thomas’ NHS Foundation Trust, London, United Kingdom Purpose or Objective Breast cancer radiotherapy has been shown to increase the risk of subsequent primary oesophageal cancer. It is unclear if avoidance of the oesophagus is being considered routinely during radiotherapy treatment planning. This study aims to describe exposure of the oesophagus from modern breast cancer regimens. Materials and Methods A systematic review of oesophageal doses from breast cancer radiotherapy regimens published 2010-2020 was undertaken. Average mean oesophageal doses and average maximum oesophageal doses were described for different anatomical regions irradiated and techniques used. Oesophageal exposure from current modern regimens was compared to that received in previous decades. Results 112 regimens from 18 countries reporting oesophagus doses were identified. The average mean oesophagus dose was 0.2 Gy (range 0.1-0.4) for partial breast irradiation, 2.7 Gy (range 0.1-16.6) for whole breast/chest wall radiotherapy and 11.4 Gy (range 0.0–31.9) with the addition of regional nodal irradiation. For regimens that included regional nodal irradiation, the average mean oesophageal dose was higher for IMRT (17.5 Gy static IMRT, 12.5 Gy rotational IMRT) than tangential radiotherapy (7.5 Gy) (p < 0.001). Overall, average oesophageal exposure from modern regimens was similar to that estimated from regimens used in previous decades. Conclusion Exposure of the oesophagus remains an issue in modern breast cancer radiotherapy particularly for patients undergoing IMRT. Routine avoidance of the oesophagus during treatment planning may reduce the number of women developing a subsequent primary oesophageal cancer in the future. PO-1129 The acute and late toxicities of MRI-guided APBI delivered using a once-per-day regimen H. Lee 1 , K.H. Shin 1 , K. Kim 2 , J.H. Kim 1 , J.H. Chang 1 1 Seoul National University College of Medicine, Department of Radiation Oncology, Seoul, Korea Republic of; 2 Ewha Womans University College of Medicine, Department of Radiation Oncology, Seoul, Korea Republic of Purpose or Objective The use of external beam accelerated partial breast irradiation (APBI) using a twice-per-day regimen has raised concerns about increase rates of late toxicities. We compared toxicity outcomes of external beam APBI using a once-per-day regimen and accelerated hypofractionated whole breast irradiation (AWBI) in patients with early-stage breast cancer. Materials and Methods This was a single-institution, retrospective cohort study. Patients aged ≥50 years with pTisN0 or pT1N0 breast cancer who underwent breast-conserving surgery and adjuvant radiotherapy were included. APBI was delivered at 38.5 Gy in 10 fractions once daily using magnetic resonance imaging (MRI)-guided radiotherapy only to patients who were strictly “suitable”, according to the ASTRO-APBI guidelines. AWBI was delivered at 40.5–43.2 Gy in 15 or 16 fractions with or without a boost. Results Between October 2015 and December 2018, 173 and 300 patients underwent APBI and AWBI, respectively. At a median follow-up of 34.9 months (range 7.1 to 55.4 months), the 3-year recurrence-free survival rates of the APBI and AWBI groups were both 99.2% (p=0.63). Acute toxicities were less frequent in the APBI than AWBI group (grade 1: 95 [54.9%] vs. 233 [77.7%] patients; grade 2: 7 [4.0%] vs. 44 [14.7%] patients; no grade ≥3 toxicities were observed in either group, p<0.001). Late toxicities were less common in the APBI than AWBI group (grade 1: 112 [64.7%] vs. 197 [65.7%] patients; grade 2: 9 [5.2%] vs. 64 [21.3%] patients; grade 3: 0 vs. 5 [1.7%] patients, p<0.001). Multivariate analysis showed that APBI was significantly associated with fewer late toxicities of grade ≥2 compared with AWBI (odds ratio 4.17, p=0.006). Conclusion Once-per-day APBI using MRI-guided radiotherapy afforded excellent locoregional control and fewer toxicities compared with AWBI. This scheme could be an attractive alternative to AWBI in patients who meet the ASTRO-