ESTRO 2021 Abstract Book

S155

ESTRO 2021

and 2016. Data collection included demographic features, tumor characteristics, treatment type, and LRR events. Analysis was restricted to patients age 50 or older with grade 1-2, ER+, HER2-, LVI -, margin - disease. Five-year LRR rates were compared across groups using Cox proportional-hazard model. Results Seven hundred patients met inclusion criteria. Of these, 570 were treated with EBRT with sequential boost to a median dose of 52.6Gy (range, 47.4-66.4Gy) including boost to a median total dose of 10.0Gy (range, 4.0- 20.0Gy), and 130 were treated with EBRT without boost to a median total dose of 42.6Gy (range 38.0-50.4Gy). Demographic and tumor variables are listed below. Boost (n=570) No Boost (n=130) Median follow-up (months) 61.5 (91-143) 51.5 (2-146) Age (median) 61 (50-82) 68 (50-87) Side or primary tumor Left breast 303 (53%) 62 (48%) Right breast 267 (47%) 68 (52%) Stage T1 507 (89%) 120 (92%) T2 63 (11%) 10 (8%) Endocrine therapy 529 (93%) 111(85%)

The 5-year rates of LRC in both boost and no boost groups were 100%. The effect of boost vs. no boost on LRC did not show statistical significance (HR 0.174; 95% CI 0.011-2.778; p=.216, Fig. 1).

Conclusion In this large modern series, patients with low risk early breast cancer treated with endocrine therapy and EBRT exhibited exceptionally high LRC, which was not improved with the addition of a boost. Meanwhile, the boost adds time, expense, and modest toxicity. Omission of a boost should be encouraged in this population. PH-0223 Pathological complete response after preoperative chemoradiotherapy for HER2+/TN breast cancer R. Ciervide 1 , A. Montero 1 , M. García-Aranda 1 , B. Alvarez 1 , X. Chen-Zhaoi 1 , R. Alonso 1 , M. Lopez 1 , O. Hernando 1 , E. Sanchez 1 , J. Valero 1 , M. Nuñez 1 , M. Izquierdo 1 , K. Rossi 1 , C. Cañadillas 1 , M.A. De la Casa 1 , J. Marti 1 , L. Alonso 1 , P. Fernandez Leton 1 , C. Rubio 1 1 HM Hospitales, Radiation Oncology, Madrid, Spain Purpose or Objective Pathologic complete response (pCR) after primary systemic treatment (PST) is associated with a significant survival gain in patients with triple negative (TN) or HER2+ breast cancer. We propose a neoadjuvant concurrent radio-chemotherapy protocol to increase rates of pCR that could be associated with an improvement in final outcomes. Materials and Methods This is a unicentric prospective IRB-approved study that includes localized TN/HER2+ tumors (cT2-4cN0-3) to receive concomitant neoadjuvant radiochemotherapy. 18 FDG-PET-CT is used for radiotherapy simulation. Radiotherapy schedule consists of 40.5Gy in 15 fractions of 2.7Gy delivered to whole breast (WBI) and axillary levels I-IV (AL) with ipsilateral internal mammary chain (IMC) when indicated and a simultaneous integrated boost of 3.6Gy in 15 fractions on macroscopic and hypermetabolic tumor areas highlighted by PET up to a total dose of 54Gy. Catalyst® SGRT system and kV-CBCT is used for IGRT allowing online inter and intrafraction patient tracking. Radiation is delivered during the first 3 weeks of chemotherapy. Concomitant chemotherapy according to molecular subtype is Pertuzumab-Trastuzumab-Paclitaxel followed by anthracyclines in Her-2 positive patients and CBDCA-Paclitaxel followed by anthracyclines in TN patients (Figure 1). Study endpoints are to assess pCR rates and tolerance.