ESTRO 2022 - Abstract Book

S825

Abstract book

ESTRO 2022

Netherlands; 9 Institute for Radiation Oncology RISO, Radiation Oncology, Deventer, The Netherlands; 10 Sant Anna Hospital, Radiation Oncology, Como, Italy; 11 University Hospital, Tübingen, Radiation Oncology, Tübingen, Germany; 12 University Hospital Zurich, Radiation Oncology, Zurich, Switzerland; 13 Medisch Spectrum Twente, Radiation Oncology, Enschede, The Netherlands; 14 Institut Curie, Radiation Oncology, Paris, France; 15 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands; 16 Leiden University Medical Centre, Radiation Oncology, Leiden, The Netherlands; 17 Iridium Netwerk and University of Antwerp, Radiation Oncology, Wilrijk-Antwerp, Belgium Purpose or Objective The multicentre EORTC 22922/10925 trial, including between 1996 and 2004 a total of 4004 stage I-III breast cancer patients with involved axillary nodes and/or a medially located primary tumour, showed a significant reduction of breast cancer specific mortality (BCSM) and any recurrence, not translating in improved overall survival (OS), and low absolute rates of side effects at 15.7 years of follow-up. The trial had a specific pre-planned radiation therapy (RT) quality assurance (QA) programme. The trial was conceived after long-term negative effects of older RT techniques became available but before the introduction of modern RT techniques. The aim of the current analysis was to evaluate the association of RT techniques used for internal mammary and medial supraclavicular (IM-MS) lymph node irradiation on long-term outcomes. Materials and Methods An exploratory and descriptive analysis of outcomes was conducted separately for the three RT techniques allowed in the trial: standard [RT based on a fixed set-up combining photon/electron beams to the IM and tangential fields to the breast or chest wall (electrons allowed as well), relative weight of mixed beam, energy and prescription depth based on the assumption that the IMNs lie within 4 cm lateral to the midline, and extends up to a depth of 4 cm] vs. standard-modified [similar to standard with minor adaptation for gantry angle; match line setting; proportion and energy of electrons] vs. individualised technique [mandate individual localisation of IMN for each patient, based on imaging in aim to adapt the beam energies, prescription depths and relative beam weights] as substantiated by the QA procedures. Techniques used were fixed per institution over the duration of the trial. Results The number of patients treated by each technique was 2440 (61%) by standard vs. 635 (16%) by standard-modified vs. 929 (23%) patients by individualised technique. The improvements of oncological outcomes in terms of disease-free survival (DFS), OS and BCSM with IM-MS RT compared to no IM-MS RT were larger for patients treated with an individualised technique (Table 1). The increase in 15-year rates of side effects due to IM-MS RT, both scored longitudinally and cross- sectionally, were similar among the techniques (Table 1). Table 1: Oncological outcomes and lung and heart toxicity at 15 years according to radiation technique, comparing IM- MS RT to no IM-MS RT OS=overall survival; DFS=disease free survival; Breast Cancer Specific =BCS

Conclusion Even though a straightforward comparison by technique is not possible because of variations in tumour-, patient- and other treatment-related characteristics between institutions, our findings suggest that the use of more individualised RT techniques is associated with higher rates of oncological improvements without increased risks for late side effects.