ESTRO 38 Abstract book

S391 ESTRO 38

3 Institute for cancer research- prevention- and clinical network ISPRO, Cancer Risk Factors and Lifestyle Epidemiology Unit, Florence, Italy ; 4 Ewha Womans University College of Medicine, Department of Radiation Oncology, Seoul, Korea Republic of Purpose or Objective Whole breast irradiation (WBI) ± boost on the primary tumour bed is the standard of care after breast-conserving surgery (BCS). For decades, conventionally fractionated WBI (CF-WBI) has been widely used. Pivotal phase 3 trials on hypofractionated-WBI (HF-WBI) showed comparable results in terms of efficacy with lower rates of acute side effects in favour of hypofractionation. However, due to the relatively low proportion of younger patients enrolled in these studies, HF-WBI is not broadly adopted for these patients. The aim of this retrospective case-control study is to confirm the safety of hypofractionation in younger patients. Material and Methods Between 2007 and 2016, a total of 786 patients aged less than 60 years old with early stage breast cancer was treated with WBI after BCS in three leading breast cancer centres: 340 underwent HF-WBI while 446 cases were treated with CF-WBI. All patients were homogenously scored following the acute and late RTOG/EORTC scales by treating expert physicians. Acute side effects were evaluated by scoring the maximum grade of oedema, erythema/pigmentation, and wet desquamation at the end of the WBI. Late toxicity was evaluated by scoring the maximum grade of oedema, erythema/pigmentation, desquamation, and breast fibrosis at 6, 12, 24, and 36 months. Results At univariate analyses, hypofractionation showed a significant protective effect in terms of acute oedema (p=0.0001), acute wet desquamation (p=0.009), chronic oedema (p=0.0001), chronic erythema/pigmentation (p=0.0001), and chronic fibrosis (p=0.0002). Main results are summarized in Figure 1 . At multivariate analysis independent factors for acute oedema were hypofractionation (HR 0.09, 95%CI 0.02 to 0.48; p=0.005), and adjuvant chemotherapy (HR 2.09, 95%CI 1.10 to 3.97; p=0.024); independent factors for chronic fibrosis were breast volume (HR 2.64, 95%CI 1.50 to 4.65; p=0.001), extensive intraductal component (HR 2.15, 95%CI 1.17 to 3.98; p=0.014), and tumour grade (HR 0.29, 95%CI 0.11 to 0.74; p=0.01). Major results are summarized in Figure 2 .

Conclusion HF-WBI showed significantly better outcome in terms of main acute and late skin side effects. Longer follow-up is needed to confirm efficacy results. In line with recently updated international guidelines, the use of HF-WBI after BCS should be strongly encouraged and may largely replace CF-WBI independently of age. PO-0762 Low predictive value of mean heart dose for coronary artery dosimetry in breast cancer radiotherapy S. JACOB 1 , J. Camilleri 2 , S. Derreumaux 3 , V. Walker 1 , O. Lairez 4 , M. Lapeyre 5 , E. Bruguière 5 , A. Pathak 6 , M. Bernier 1 , D. Laurier 7 , J. Ferrières 8 , D. Broggio 9 , G. Jimenez 2 1 Institut de Radioprotecton et de Sureté Nucléaire IRSN, Laboratory of Epidemiology, Fontenay-aux-Roses / Toulouse, France ; 2 Clinique Pasteur, Radiotherapy Oncorad, Toulouse, France ; 3 Institut de Radioprotecton et de Sureté Nucléaire IRSN, Unit of expertise in medical radiation protection, Fontenay-aux-Roses, France ; 4 Rangueil University Hospital, Cardiology, Toulouse, France ; 5 Clinique Pasteur, Radiology, Toulouse, France ; 6 Clinique Pasteur, Cardiology, Toulouse, France ; 7 Institut de Radioprotecton et de Sureté Nucléaire IRSN, Department of health and biological effects of ionizing radiation, Fontenay-aux-Roses, France ; 8 National Institute of Medical Research INSERM, Cardiovascular Epidemiology, Toulouse, France ; 9 Institut de Radioprotecton et de Sureté Nucléaire IRSN, Dosimetry, Fontenay-aux-Roses, France Purpose or Objective In many studies that investigated radiation-induced cardiac toxicity of breast cancer radiotherapy, doses are described as those received by the entire heart and the mean heart dose is used as the reference dose for analyzing dose-response relationship. However the specific relationships between doses to cardiac sub- structures, in particular coronary arteries, and subsequent toxicity have not been well defined. Detailed individual dosimetry information for the heart and its sub-structures is required to better understand cardiac damage from radiation exposure. The aim of this dosimetric study was to analyze the distribution of individually-determined radiation exposure, in a population of breast cancer patients treated with three dimensional conformal radiation therapy (3D-CRT), and clarify whether mean heart dose is a good surrogate parameter for the dose to coronary arteries, in particular the left anterior descending artery. Material and Methods