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Conclusion Clinical outcomes of hypofractionated RT were comparable to conventional RT in women undergoing BCT. However LRC rates were statistically inferior in patients undergoing MRM in presence of node positive disease, hormone receptor negative status and high grade (IDC grade 3) tumors. Hypofractionation should be cautiously used in this subgroup of patients. EP-1330 Dosimetric comparison of two techniques in left breast cancer with respiratory gating M. Valli 1 , I. Donegani 1 , E. Cekani 1 , D. Gaudino 2 , D. Daniele 1 , D. Bignasca 1 , A. Richetti 1 , S. Cima 1 , F. Martucci 1 , C. Azinwi 1 , G. Pesce 1 , D. Bosetti 1 , K. Yordanov 1 , I. Maddalena 1 , S. Presilla 2 1 Oncology Institute of Southern Switzerland, Department of Radiation Oncology, Bellinzona, Switzerland 2 Ente Ospedaliero Cantonale, Medical Physics Unit-, Bellinzona, Switzerland Purpose or Objective Aim of this study was to compare Volumetric multiple arcs therapy (VMAT) and 3D conformal radiotherapy (3DCRT) for sparing organs at risk (OAR) in left breast cancer patients treated with post-operative RT using Deep Inspiration Breath Hold (DIBH). Material and Methods One hundred sixteen patients were treated with DIBH technique, from January 2016 to September 2017. We retrospectively analyzed 50 patients treated on whole breast plus surgical bed. The prescribed dose was 50 Gy to PTV II and 10-16 Gy to PTV I, 2 Gy/fraction. For each patient the treatment technique, 3DCRT or VMAT, has been chosen according to ICRU recommendations. Finally two homogeneous groups were compared: 24 patients treated with the 3DCRT modality and 16 with the VMAT one. The median PTVII, PTVI and OAR volumes were comparable in both groups. These dosimetric parameters have been analyzed to compare the two different techniques: Dmean heart, Dmax heart, V30 heart, V20 heart, V10 heart, Dmax LAD, Dmean ipsilat lung, D2% ipsilat lung, V30 ipsilat lung, V20 ipsilat lung, V5 ipsilat lung, Dmean contralat lung, D2% contralat lung, V5 contralat lung, Dmean contralat breast, D2% contralat breast, V5 contralat breast. A Mann-Whitney statistical test was used and statistical p value < 0.01 was considered significant. Results 3DCRT achieved a statistically significant dose reduction for the following parameters: D mean heart, V 20 heart, V 10 heart, D max LAD, D mean ipsilat lung, V 20 ipsilat lung, V 5 ipsilat lung, D mean contralat lung, D 2% contralat lung, V 5 contralat lung, D mean contralat breast, D 2% contralat breast, V 5 contralat breast. Conversely VMAT get a statistically significant advantage for D 2% ipsilat lung. 3DCRT and VMAT plans were comparable for other parameters: Dmax heart, V30 heart and V30 ipsilat lung. Table1 Conclusion In our experience 3DCRT technique allows good results in terms of OAR sparing if associated with DIBH modality. A comparison with VMAT is mandatory for improving, if the case, the PTV dose distribution homogeneity. EP-1331 Role of postoperative radiotherapy in DCIS: an observational study of 1,048 cases S. Corradini 1 , M. Pazos 1 , S. Schönecker 1 , D. Reitz 1 , M. Niyazi 1 , U. Ganswindt 1 , M. Braun 2 , N. Harbeck 3 , J. Engel 4 , C. Belka 1 1 LMU Munich, Department of Radiation Oncology, Munich, Germany 2 Red Cross Hospital, Department of Obstetrics and Gynecology- Breast Centre, Munich, Germany 3 LMU Munich, Department of Obstetrics and Gynecology- Breast Centre, Munich, Germany

18.6% (p=0,73), V28/V30: 13.7% vs 13.9% vs 14.9% (p=0.88) and the mean lung dose (EQD2): 9.7 Gy vs 9.6 Gy vs 10.6Gy (p=0,62). Conclusion This contemporary cohort study confirms that DIBH significantly reduced heart exposure in left sided breast cancer patients but does not appear to affect lung volume parameters in clinical practice. EP-1329 Clinical Outcomes Of Hypofractionated Vs Conventional Radiation In Women Treated For Breast Cancer A. Budrukkar 1 , N. Dash 1 , T. Wadasadawalla 1 , R. Jalali 1 , R. Upreti 2 , V. Parmar 3 , S. Gupta 4 , R. Badwe 3 , R. Sarin 1 1 Tata Memorial Hospital-, Department of Radiation Oncology - Tata Memorial Hospital, Mumbai, India 2 Tata Memorial Hospital, Department of Medical Physics, Mumbai, India 3 Tata Memorial Hospital, Department of Surgery, Mumbai, India 4 Tata Memorial Hospital, Department of Medical Oncology, Mumbai, India Purpose or Objective Hypofractionation for breast cancer has been proven be equivalent to conventional RT in various randomized trials. The aim of this study is to compare the two different fractionation regimens used in breast conserving therapy (BCT) and mastectomy (MRM) over a period of 2 years- 2013 when conventional fractionation was used vs 2014 when hypofractionation was adopted in our institution. During this 2 year period 1288 women were treated with RT. Current data is an interim analysis of 520 women. Material and Methods Five hundred and twenty patients who were treated with radiation therapy from January 2013 (n=272) to December 2014 (n=248) with histologically proven invasive breast cancer were analyzed in this study. Patients were treated with conventional fractionation (early breast cancer:45Gy/25 fr, locally advanced: 50Gy/25 fractions) while MRM patients were treated to a dose of 45Gy/20 fractions. From January 2014 we adopted hypofractionation (40Gy/15 fractions) for both BCT and MRM. Boost was given to all patients undergoing BCT. Data was extracted from electronic medical records and radiation oncology information system. Data was entered using SPSS software for windows (v21). Survival analysis was done using Kaplan Meier test. Log rank test was used to compare various outcomes. Results Median follow up was 25 months. Three year the local control (LC) was 94.1% for the entire group, locoregional control (LRC) was 90.2%, disease free survival (DFS) was 76.5% and overall survival (OS) was 92.5%. In women undergoing BCT the 3 year LC in conventional and hypofractionated group was 97% and 97.6% (p=0.613), LRC was 94.9% and 95.6% (p= 0.894), DFS was 88.6 % and 90.4% (p=0.905), and OS was 97% and 98.1% (p=0.574) respectively. There was no difference in LRC in women undergoing BCT with conventional and hypofractionated radiotherapy even in presence of node positivity, presence of LVE, higher grade (IDC 3) and hormone receptor negative disease. In patients who underwent MRM the LC was 93% and 88.2% (p=0.077), LRC was 89.5% and 83.9% (p=0.017), DFS was 63.5% and 57.5% (p=0.8) and OS was 86.5% and 92% (p=0.476) in mild and moderate hypofractionation group respectively. At 3 year the LRC was 86.7% and 79.3% in node positive (p=0.018), 85.2% and 71.8% in hormone receptor negative (95% CI- 36.4-42.4, p=0.013), 87.6% and 78.6% (95% confidence interval [CI] 38.1-41.8, p=0.011) in IDC grade III tumor, and 85% and 63% in presence of LVE (p=0.113) in mild and moderate hypofractionation group respectively.