ESTRO 2023 - Abstract Book

S630

Monday 15 May 2023

ESTRO 2023

Oncology, Dijon, France; 8 Comprehensive Cancer Center, Charite University Medicine, Department of Radiation Oncology , Berlin, Germany; 9 University Medical Centre Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands; 10 RISO, Deventer, Institute for Radiation Oncology , Deventer, The Netherlands; 11 University Hospital, Department of Radiation Oncology, Tübingen, Germany; 12 Gustave Roussy Cancer Centre, Department of Radiation Oncology, Villejuif, France; 13 Cancer Center Amsterdam, Department of Radiation Oncology, Amsterdam, The Netherlands; 14 Institut Curie, Department of Radiation Oncology, Paris, France; 15 Centre Paul Strauss, Department of Radiation Oncology, Strasbourg, France; 16 Sant Anna Hospital, Department of Radiation Oncology, Como, Italy; 17 University Hospital Zurich, University of Zurich, Department of Radiation Oncology, Zurich, Swaziland ; 18 Medisch Spectrum Twente, Department of Radiation Oncology, Enschede, The Netherlands; 19 Centre Léon Bérard, Department of Radiation Oncology, Lyon, France; 20 Rambam Medical Centre, Department of Radiation Oncology, Haifa, Israel; 21 Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands; 22 Leiden University Medical Centre, Department of Radiation Oncology, Leiden, The Netherlands; 23 Iridium Netwerk, University of Antwerp, Faculty of Medicine and Health Sciences, Department of Radiation Oncology, Wilrijk-Antwerp, Belgium Purpose or Objective To investigate local recurrence (chest wall or in-breast, LR) rate, including spatial distribution, from the EORTC 22922/10925 trial. Materials and Methods The multicentre EORTC trial randomised stage I-III breast cancer patients with involved axillary nodes and/or a medially located primary tumour between internal mammary and medial supraclavicular radiation (IM-MS RT) or no IM-MS RT. Primary surgery included mastectomy or breast conservation surgery (BCS) followed by whole breast RT. Chest wall RT after mastectomy or tumour bed boost in case of BCS were left to the discretion of the treating physician. Postoperative systemic therapy was given as defined by institutional guidelines. An exploratory analysis of the effect of treatment on LR rate was conducted after a median follow-up of 15.7 years using the Fine & Gray model accounting for competing risks and adjusted for baseline patient and disease characteristics. The significance level was set at 5% 2-sided. Spatial location of LR was defined according to the trial’s case report forms (CRF) and presented in descriptive statistics. Results The current analysis includes 3,049 patients who underwent BCS, of which 3039 (99.7%) received whole breast RT, in 2,597 (85.2%) patients followed by a boost to the primary tumour bed and a total of 955 patients who underwent mastectomy (23.9%), with chest wall RT in 701 patients (73.4%, equal in both study groups). Cumulative incidence rate (CIR) of LR at 15 years was lower after mastectomy (3.1%) compared to BCT (7.3%) (F&G: HR (Hazard Ratio) =0.421, 95%CI=0.282-0.628, p- value<0.0001) (Figure 1). CIR at 15 years was 7.5% for patients who received chemotherapy (F&G: HR=0.685, 95%CI=0.441- 1.064, p-value=0.092), 4.7% for endocrine therapy-only (F&G: HR=0.555, 95%CI=0.376-0.818, p-value=0.0029), 4.4% for both (F&G: HR=0.449, 95%CI=0.283-0.713, p-value=0.0007), compared to 10.7% CIR in the group that did not receive any systemic therapy. There was no significant interaction between type of local therapy and postoperative systemic therapy and no significant effect of IM-MS RT on local recurrence. Table 1 shows the spatial location of LR, showing that the majority of the recurrences occurred outside of the primary tumour bed.