ESTRO 2021 Abstract Book

S913

ESTRO 2021

(80%), with no G3 events described. Breast pain was reported in 3 cases (12%). Blood tests showed 3 cases of G1 leucopenia (12%) and 1 case of G1 anemia (4%); 6 patients had thrombocytopenia (24%), with 3 cases (12%) of ≥ G2 toxicity. Nine cases (36%) of elevation of liver transaminases were reported, with 1 G2 (4%) and 1 G3 (4%) toxicities. Treatment with TDM-1 was interrupted in 4 patients (16%) due to the onset of thrombocytopenia or elevation of liver transaminases. Left ventricular ejection fraction (LVEF) remains stable between TDM-1 start and the end of RT with an average reduction of 0.79 point% (SD + 3.9), only 1 patient registered an asymptomatic LVEF reduction >10%.

Conclusion The acute toxicity rate, especially focusing on skin and cardiac adverse events, were assumed acceptable in our cohort. To safely administer this concomitant treatment. Further evidence coming from larger series and prospective experiences are needed. PO-1097 Preclinical evaluation for dose escalation in the preoperative SBRT of early breast cancer C. Piccolo 1 , M.A. Zerella 2 , E. Rondi 1 , S. Vigorito 1 , A.M. Ferrari 2 , P. Veronesi 3,4 , V. Galimberti 3 , R. Orecchia 5 , F. Cattani 1 , B.A. Jereczek-Fossa 2,6 , M.C. Leonardi 2 1 IEO European Institute of Oncology, IRCCS, Unit of Medical Physics, Milan, Italy; 2 IEO European Institute of Oncology, IRCCS, Division of Radiotherapy, Milan, Italy; 3 IEO, European Institute of Oncology, IRCCS, Division of Breast Cancer Surgery, Milan, Italy; 4 University of Milan, Faculty of Medicine, Milan, Italy; 5 IEO, European Institute of Oncology IRCCS, Scientific Directorate, Milan, Italy; 6 University of Milan, Department of Oncology and Hemato-oncology, Milan, Italy Purpose or Objective This investigation was developed within the “Phase I/II clinical trial on single fraction ablative pre-operative radiation treatment (RT) for early-stage breast cancer” study (AIRC IG 2019 - ID. 23118 project). The purpose of the research project is to investigate the effect of preoperative high-dose single fraction to primary breast cancer in order to assess pathological response. A preclinical study was performed with the aim of exploring the feasibility of three different prescription doses (18 Gy, 21 Gy and 24 Gy) to be delivered with CyberKnife Stereotactic Body Radiation Therapy (SBRT) (Accuray Inc., Sunnyvale, CA). The dosimetric results were compared with the constraints for Organs at Risk (OARs) according to Timmerman for single fraction thorax SBRT ( Stereotactic ablative body radiosurgery (SABR) or Stereotactic body radiation therapy (SBRT). Folkert MR, Timmerman RD. Adv Drug Deliv Rev. 2017 Jan 15 ;109:3-14 ). Materials and Methods