ESTRO 2020 Abstract book

S564 ESTRO 2020

⩽ 2 mm but negative, RT boost was 51 Gy

surgical margins in 15 fractions. Results

Median age was 47 years (range 23-75 years). All patients had ECOG 0–1. The patient- tumor and treatment-related characteristics are summarized in tables 1 and 2. Clinical stage was 0is in 32 patients (6.9%), I in 242 (51.6%), II in 171 (36.5%) and III in 24 (5%). 119 patients (25.4%) underwent regional RT. Estrogen-receptor status was positive in 364 (77.6%) patients. 314 patients (67%) received chemotherapy and 350 (74.5%) hormonotherapy. Median PTV boost volume was 61cm3 (range 12-822cm3) y median PTV breast volumen was 747 cm3 (range 173-2686cm3).The acute skin toxicity was mild in 346 patients (73.8%), moderate in 98 (20.9%) and severe in 1 patient (0.2%). 53 patients (11.3%) had grade 1 or 2 late toxicity (fibrosis) to the boost area. One patient (0.2%)had grade 3 fibrosis and underwent mastectomy. At a median follow-up of 47 months, local control was 97.5%. In the multivariate analysis, fibrosis was significantly associated with the PTV breast volume (p=0.01) greater than 747cm3 and PTV boost volume (p=0.01) greater than 61cm3.

Conclusion HFRT with concomitant boost allows for treatment acceleration and dose escalation in the area of higher risk of relapse. According to the findings from this study, HFRT with concomitant boost appears to be a safe and effective technique, with acceptable toxicity. PO-0973 Axillary lymph node dissection after neoadjuvant chemotherapy for node-positive breast cancer. M. Van Zeeland 1 , P. Westhoff 1 , C. Wauters 2 , P. Bult 3 , A. Werner 4 , N. Laurens 5 , L. Strobbe 6 , H. Meijer 1 1 Radboud University Medical Center, Radiation Oncology, Nijmegen, The Netherlands ; 2 Canisius Wilhelmina Hospital, Pathology, Nijmegen, The Netherlands ; 3 Radboud University Medical Center, Pathology, Nijmegen, The Netherlands ; 4 Radboud University Medical Center, Surgical Oncology, Nijmegen, The Netherlands ; 5 Canisius Wilhelmina Hospital, Medical Oncology, Nijmegen, The Netherlands ; 6 Canisius Wilhelmina Hospital, Surgical Oncology, Nijmegen, The Netherlands Purpose or Objective Following neoadjuvant chemotherapy (NAC) for clinically node-positive breast cancer, axillary lymph node dissection (ALND) is often performed, when the marked node remains positive after NAC. The purpose of this study was to determine the possibility of predicting additional positive lymph nodes in ALNDs and to identify potential risk factors for having additional positive lymph nodes in ALNDs. Material and Methods The selected patients had been diagnosed with invasive breast cancer and lymph node positive disease detected by ultrasound of the axilla and cytologically or histologically confirmed (cT1-4 N1-3 M0). The positive lymph nodes were marked with a radioactive iodine seed or with a radio- opaque marker. Patients with a persistently positive marked lymph node after NAC, undergoing ALND, were analyzed for predictors of additional positive lymph nodes in ALNDs. Results