ESTRO 2021 Abstract Book

S902

ESTRO 2021

PO-1084 Radiotherapy with simultaneous integrated boost for breast cancer: acute toxicity outcomes I. Ratosa 1,2 , M. Anzic 1 , A. Gojkovic Horvat 1 , M. Ivanetic Pantar 1 , D. Golo 1 , J. Gugic 1 , M.S. Paulin Kosir 1 , T. Marinko 1,2 1 Institute of Oncology Ljubljana, Division of Radiation Oncology, Ljubljana, Slovenia; 2 University of Ljubljana, Faculty of Medicine, Ljubljana, Slovenia Purpose or Objective To prospectively evaluate and compare acute adverse events (AEs) of hypofractionated and normofractionated adjuvant radiotherapy (RT) with simultaneous integrated boost (SIB) after breast-conserving surgery (BCS). Materials and Methods Patients with stage 0–III breast cancer (BC) with an indication for a tumor bed boost as a part of adjuvant RT to the whole breast with or without locoregional RT following BCS were eligible for this prospective observational study. Patients were treated with either conventionally fractionated (CF-SIB) or hypofractionated (HF-SIB) RT. With conventional fractionations whole breast received a dose of 50 Gy in 25 fractions or 51.52 Gy in 28 fractions and tumour bed received a boost dose of 57 Gy or 63 Gy, respectively. For hypofractionated RT four different dose prescriptions were allowed. Whole breast was irradiated with 40.05 Gy in 15 fractions, 42.3 Gy in 18 fractions, 45.57 Gy in 21 fractions or 46.69 Gy in 23 fractions. Tumour bed received 45.75 Gy in 15 fractions, 52.2 Gy in 18 fractions, 55.86 Gy in 21 fractions or 61.18 Gy in 23 fractions, respectively. RT was given either as 3D conformal RT (3DCRT) or as intensity-modulated RT (IMRT). AEs were prospectively evaluated during RT weekly or bi-weekly. Grading was performed in analogy with the Common Terminology Criteria for Adverse Events version 5 (CTCAE v.5.0). Maximum grades of AEs observed during RT were taken for the analysis. Results The study population was composed of 93 female patients with a median age of 50 years that were treated between 2018 and 2020. Sixty-five (69.9%) patients were treated using IMRT and 28 (30.1%) patients were treated using 3DCRT. Twenty-three (24.7%) patients received CF-SIB and 70 (75.3%) patients received HF-SIB. Overall, grade 1 (G1) acute skin toxicity was observed in 70 (75.3%) patients, G2 in 12 (12.9%) patients and G3 in 1 (1.1%) patient. Radiodermatitis was the only G3 toxicity detected. We found no statistical difference in acute skin toxicity rates comparing CF-SIB or HF-SIB schedules (Figure 1) (p=0.143). Furthermore, there was no statistically significant difference in terms of any grade acute skin toxicity according to the RT technique (3DCRT or IMRT; p=0.921). Compared to patients treated with CF-SIB, patients treated with HF-SIB reported similar rates of G1/G2 fatigue (69.6% vs. 69.6 %; p=0.072). We observed no influence of the different RT schedules on the occurrence of the following G1/G2 acute side effects at the treated area: pruritus (56.5% vs 41.4; p=0.228), sensibility changes (43.5% vs 41.4%; p=0.227) breast oedema (34.7% vs 22.9%; p=0.257) and reported any pain (56.5% vs 50.0%; p=0.111) in patients treated with CF-SIB vs HF-SIB, respectively.

Conclusion Our data confirmed that adjuvant RT with SIB is well tolerated in terms of acute side effects. Both CF-SIB and HF-SIB regimens proved to be comparable however the latter considerably shorten adjuvant RT times without compromising the rates of acute toxicity. PO-1085 Longitudinal assessment of immune infiltrate in breast cancer treated with neoadjuvant radiotherapy R. Robinson 1,2 , I. Roxanis 3 , F. Sobhani 4,5 , K. Zormpas-Petridis 1 , H. Steel 1 , S. Anbalagan 1 , A. Sommer 1 , L. Gothard 1 , A. Khan 1,2 , F. MacNeill 2 , A. Melcher 1,6 , Y. Yuan 4,5 , N. Somaiah 1,2 1 Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom; 2 Royal Marsden Hospital NHS Foundation Trust, Breast Unit, London, United Kingdom; 3 Institute of Cancer Research, The Breast Cancer Now Toby Robins Research Centre, London, United Kingdom; 4 Institute of Cancer Research, Molecular