ESTRO 2021 Abstract Book

S41

ESTRO 2021

cancer.

Proffered papers: Proffered papers 3: Breast

OC-0067 De-escalation of radiation therapy after primary chemotherapy in cT1-2N1 breast cancer (RAPCHEM) S. de Wild 1 , L. de Munck 2 , J. Verloop 2 , T. van Dalen 3 , P. Elkhuizen 4 , R. Houben 5 , E. van Leeuwen 6 , S. Linn 7 , R. Pijnappel 8 , P. Poortmans 9 , L. Strobbe 10 , J. Wesseling 11 , A. Voogd 2,12 , L. Boersma 5 1 Maastricht University Medical Centre+, Dept. of Surgery, GROW School for Oncology and Developmental Biology, Maastricht, The Netherlands; 2 Netherlands Comprehensive Cancer Organisation, Dept. of Research and Development, Utrecht, The Netherlands; 3 Diakonessen Hospital, Dept. of Surgery, Utrecht, The Netherlands; 4 Antoni van Leeuwenhoek - The Netherlands Cancer Institute, Dept. of Radiation Oncology, Amsterdam, The Netherlands; 5 Maastricht University Medical Centre+, Dept. of Radiation Oncology (Maastro), GROW School for Oncology and Developmental Biology, Maastricht, The Netherlands; 6 Dutch Breast Cancer Research Group, BOOG, Amsterdam, The Netherlands; 7 Antoni van Leeuwenhoek - The Netherlands Cancer Institute, Dept. of Medical Oncology, Amsterdam, The Netherlands; 8 University Medical Centre Utrecht, Dept. of Radiology, Utrecht, The Netherlands; 9 Iridium Network, Antwerp University, Dept. of Radiation Oncology, Antwerp, Belgium; 10 Canisius Wilhelmina Hospital, Dept. of Surgery, Nijmegen, The Netherlands; 11 Antoni van Leeuwenhoek - The Netherlands Cancer Institute, Dept. of Pathology, Amsterdam, The Netherlands; 12 Maastricht University Medical Centre+, Dept. of Epidemiology, GROW School for Oncology and Developmental Biology, Maastricht, The Netherlands Purpose or Objective Over the past decades, primary chemotherapy (PCT) has increasingly been applied in breast cancer (BC) patients. This poses a dilemma with regard to postoperative radiation therapy (RT), since locoregional RT (LRRT) guidelines were originally based on pathological status after primary surgery. As a result, it is now unclear when LRRT is indicated after PCT, especially in cT1-2N1 patients. We hypothesized that LRRT can be de-escalated based on ypN-status. Therefore, a prospective cohort study (RAPCHEM, NCT01279304, BOOG 2010-03) was started to assess locoregional control after LRRT, with a predefined consensus-based study guideline (SG). We hereby present the 5-year results for locoregional recurrence (LRR) rates, recurrence-free interval (RFi), and overall survival (OS). Materials and Methods Seventeen Dutch RT centres included cT1-2N1 (unless ≥4 suspicious nodes on imaging) BC patients from January 2011 to January 2015. Patients were eligible if they received PCT and surgery of the breast and axilla. Based on ypN-status, they were assigned to one of three risk-groups for LRR, as defined in the SG, with accompanying recommendations: (1) ypN0 (low-risk), whole breast RT (WBRT) after lumpectomy, no RT after mastectomy; (2) ypN1 (intermediate-risk), WBRT after lumpectomy, RT of the chest wall after mastectomy, RT of axillary levels 1-2 if only a sentinel lymph node biopsy (SLNB) was performed; (3) ypN2 (high-risk), WBRT after lumpectomy, RT of the chest wall after mastectomy, RT of the non-resected part of the axilla (level 3-4 after axillary lymph node dissection, and level 1-4 after SLNB). RFi was defined as the time between primary BC diagnosis until local, regional, or distant recurrence, or death from BC, whichever came first, measured in days. We performed Kaplan-Meier survival analyses to assess LRR rates, RFi, and OS, and the log rank-test to compare the outcomes per risk group, taking into account adherence to the SG. Results A total of 848 patients were included: 292 in the low-risk group, 374 in the intermediate-risk group, and 182 in the high-risk group. Ten patients were lost to follow-up. Forty-three patients had a LRR (5.1%), of which 25 patients had synchronous distant metastases (58.1%). The results are presented in Table 1. In all risk groups, the 5-year LRR rate was less than 4%. In the low-risk and intermediate-risk group, less or more RT than prescribed in the SG did not result in a significantly altered LRR rate, RFi, or OS. In the high-risk group, less RT than prescribed in the SG resulted in a significantly higher LRR rate.