ESTRO 2020 Abstract book

S556 ESTRO 2020

regional (nodal) relapse-free survival was 98.7%, and distant relapse-free survival was 96.6%. The 5-year Kaplan- Meier estimate of overall survival was 97.3% and cancer specific survival 98.9%. Table 1. Local relapse according to molecular subtypes Histology Local relapse (%) in-situ carcinoma 4.23 % Luminal A 0.8%

followed by WBI (60Gy/30f). Supraclavicular and axillary areas were irradiated in both groups when clinically indicated (46-50Gy/23-25f).All patients received systemic therapy (chemotherapy, hormonotherapy or immunotherapy).The median follow-up was 61 months (range:28-135 months). Recurrence at distance occurred in two patients of each group. Only one of those women (no breast surgery group) is alive. No locorregional recurrences were reported. No significant differences were observed between the mastectomized group and no breast surgery group in recurrence (22% vs. 9%, p=0.719), 5-year DFS (88.9% vs. 95%,p=0.129) or 5-year OS (88.9% vs. 95.2%,p=0.119). Patients with ≤3 metastatic lymph nodes had longer 5-year DFS than those with >3 metastatic lymph nodes (100% vs. 80.8%, p=0.004). Conclusion This study suggests that the OBC with axillary lymph node metastasis can be treated with ALND without breast surgery followed by WBI, since it shows no inferiority when compared with mastectomy. The number of metastatic lymph nodes is the most important predictive factor. Further studies with randomization and longer follow-up are needed. PO-0955 Molecular subtypes and local control in 1054 breast cancer patients treated with de-escalated 3DCRT A. Fodor 1 , P. Mangili 2 , C. Brombin 3 , F. Zerbetto 1 , B. Longobardi 2 , F. Borroni 1 , R. Tummineri 1 , M. Pasetti 1 , P. Rancoita 3 , L. Perna 2 , I. Dell'Oca 1 , C.L. Deantoni 1 , P.G. Esposito 2 , A.M. Deli 1 , E. Rossi 2 , A. Chiara 1 , S. Broggi 2 , N. Slim 1 , P. Passoni 1 , M. Cattaneo 2 , A. Bolognesi 1 , C. Fiorino 2 , M.S. Di Serio 3 , N.G. Di Muzio 4 1 IRCCS San Raffaele Scientific Institute, Department of Radiotherapy, Milan, Italy ; 2 IRCCS San Raffaele Scientific Institute, Medical Physics, Milan, Italy ; 3 Vita- Salute San Raffaele University, University Center for Statistics in the Biomedical Sciences, Milan, Italy ; 4 Vita- Salute San Raffaele University, Department of Radiotherapy, Milan, Italy Purpose or Objective To report 5-year outcomes in low-risk breast cancer (BCA) patients (pts) treated with breast conservative surgery (BCS) and whole-breast hypofractionated (WBH) adjuvant multi-segment 3DCRT without boost. Material and Methods From February 2009 to March 2018 1054 pts with pTis-pT2 (≤ 3 cm), pN0-N1aM0 BCA were treated with WBH multi- segment 3DCRT, to a total dose of 40 Gy/15 fractions delivered in 3 weeks, without boost. Median age was 62 (28-90) years. Seven % of pts had ductal carcinoma in situ (CDIS), 7.6% lobular invasive carcinoma (CLI), 74.1 % ductal invasive carcinoma (CDI), 3% mixed invasive carcinoma (CLI+CDI) and 8.3% other histology (mucinous, tubular, papillary, etc.). Right sided tumors were 45.8% and left sided 54% (0.2% bilateral). Chemotherapy was prescribed in 28.4% of pts, hormonal therapy in 80.3% of pts. Histological subtypes were: 7% in situ, 47.5% Luminal A, 28.6% Luminal B Her 2 neu negative, 5.3% Luminal B Her 2 neu positive, 4.1% Erb 2 overexpression, and 7.5% Basal like. Toxicity was evaluated with RTOG scale (acute) and SOMA-LENT score (late). Results Median follow up was 67.9 (interquartile range: 45-93) months. A median number of 4 (2-6) segments with wedges were used and bolus was admitted for the first 500 pts to obtain a homogeneous dose distribution also at the skin level. Acute toxicity was: 16.8 % G0, 70.2% G1, 12.1 % G2, 0.9% G3. Late toxicity was divided into edema- hyperpigmentation (67% G0, 30.8% G1, 2.1% G2, 0.1% G3) and atrophy-fibrosis-telangiectasia-pain (88% G0, 10.2% G1, 1.2% G2, 0.5% G3). For the local recurrence rate (LR) according to molecular subtypes see Table 1. In pts younger than 50 years the LR was 3.39%. Five-year Kaplan- Meier estimate of local relapse-free survival was 97.9%,

Luminal B Her 2 neu negative 2.97% Luminal B Her 2 neu positive 5.36% Erb B2 overexpression 11.63% Basal like 3.8%

Conclusion The de-escalated (without boost) breast cancer radiotherapy after BCS, with WBH multi-segment 3DCRT demonstrated low acute and late toxicity ≥ G2. Local control is good for Luminal A and Luminal B Her 2 negative pts but in young age, ERb overexpression and Basal like molecular subtypes 5-year LR without boost is high. Our protocol was modified to include a simultaneous integrated boost in these pts. PO-0956 Neuroendocrine Tumors of the Breast: an international series of the Rare Cancer Network N. Grellier-Adedjouma 1 , A. Paix 1 , P. Franco 2 , Y.M. Kirova 3 , B. De Bari 4 , D. Pasquier 5 , H. Meijer 6 , D.C. Oksüz 7 , K. Peignaux-Casasnovas 8 , F. Huguet 9 , A. De Caluwé 10 , K. Khanfir 11 , A. Vargas 12 , J. Thariat 13 , J.A. Vargo 14 , E.M. Ozsahin 15 , Y. Belkacemi 1 1 Hopital Henri Mondor, Oncology Radiotherapy, Creteil, France ; 2 Università degli Studi di Torino, Oncology Radiotherapy, Torino, Italy ; 3 Institut Curie, Oncology Radiotherapy, Paris, France ; 4 CHRU Jean Minjoz, Oncology Radiotherapy, Besançon, France ; 5 Centre Oscal Lambret, Oncology Radiotherapy, Lille, France ; 6 Radboudumc, Oncology Radiotherapy, Nijmegen, The Netherlands ; 7 University-Cerrahpaşa, Oncology Radiotherapy, Istanbul, Turkey ; 8 Centre Georges François Leclerc, Oncology Radiotherapy, Dijon, France ; 9 Hopital Tenon, Oncology Radiotherapy, Paris, France ; 10 Institut Jules Bordet, Oncology Radiotherapy, Bruxelles, Belgium ; 11 Hôpital du Valais, Oncology Radiotherapy, Sion, Switzerland ; 12 Instituto de radiomedicina, Oncology Radiotherapy, Santiago, Chile ; 13 Centre François Baclesse, Oncology Radiotherapy, Caen, France ; 14 West Virginia University, Oncology Radiotherapy, Morgantown, USA ; 15 Centre Hospitalier universitaire Vaudois, Oncology Radiotherapy, Lausanne, Switzerland Purpose or Objective Primary neuroendocrine tumors (NET) of the breast are rare breast tumor, representing less than 1% of breast cancers.Their prognosis appears to be poorer than that of classical invasive breast carcinomas, with shorter OS and DFS at equal stage. There are no specific therapeutic guidelines for those tumors. The aim of this study was to analyze the histological, prognostic and therapeutic specificities of this rare disease. Material and Methods All women with primary NET of the breast from 14 different centers and 8 countries between 1995 and 2015 were included. Clinical, pathological and therapeutic data were collected retrospectively. Statistical analyses were performed using R v3.4.3 . Survival analysis was determined using the Kaplan-Meier method. Factors influencing overall survival and disease- free survival were first evaluated with a univariate Cox analysis with a significant cutoff of p<0.2. Multicollinearity