ESTRO 2025 - Abstract Book

S4431

Late-breaking abstracts

ESTRO 2025

Schreiber 12 , Mechthild Krause 13 , Unn-Miriam Kasti 14 , Louise Wichmann Matthiessen 15 , Piotr Kedzierawski 16 , Tanja Marinko 17 , Miia Mokka 18 , Tanja Skyttä 19 , Maj-Britt Jensen 20 , Jens Overgaard 1 1 Department of Experimental Clinical Oncology, Aarhus University Hospital, Aarhus, Denmark. 2 Aarhus University, Clinical Institute, Aarhus, Denmark. 3 Department of Oncology, Aarhus University Hospital, Aarhus, Denmark. 4 Department of Oncology, Vejle Hospital, Vejle, Denmark. 5 Department of Oncology, Odense University Hospital, Odense, Denmark. 6 Department of Oncology, Stavanger Hospital, Stavanger, Norway. 7 Department of Oncology, Rigshospitalet, Copenhagen, Denmark. 8 Department of Radiotherapy of Oncology, Clinique Universitaites Saint-Luc, Bruxelles, Belgium. 9 Department of Oncology, Aalborg University Hospital, Aalborg, Denmark. 10 Department of Oncology, Zeeland University Hospital, Næstved, Denmark. 11 Departmen of Clinical Medicine, The Arctic University of NorwayAarhus University Hospital, Tromso, Norway. 12 Department of Oncology, Academic Teaching Hospital Dresden-FriedrichstadtAarhus University Hospital, Dresden, Germany. 13 Clinic for Radiotheapy and Oncology, University Hospital Gustav carl Gustav Carus, Technische Universitat Dresden, Dresden, Germany. 14 Department of Oncology, Hospital of kristansandversity Hospital, Kristiansand, Norway. 15 Department of Oncology, Herlev and Gentofte University Hospital, Herlev, Denmark. 16 Department of Oncology, Holycross Cancer Center, Kielce, Poland. 17 Department of Oncology, Lubljana University Hospital, Lubljana, Slovenia. 18 Department of Oncology, Turko University Hospital, Turko, Finland. 19 Department of Oncology, Tampere University Hospital, Tampere, Finland. 20 DBCG, Rigshospitalet, Copenhagen, Denmark Purpose/Objective Moderately hypofractionated breast cancer (BC) radiation therapy (RT) based on 40Gy/15fr has been increasingly used, however, not for loco-regional therapy due to concern over more morbidity. The non inferiority DBCG Skagen trial 1 hypothesised that 40Gy/15fr (2.67Gy/fr) did not result in more arm lymphedema than 50Gy/25fr (2.0Gy/fr) 3 years after RT (NCT02384733). Material/Methods Patients operated for node-positive BC were randomised 50Gy/25fr vs. 40Gy/15fr. All tumour-bed boosts were simultaneous integrated (SIB). The primary endpoint was 3-year arm lymphedema (≥10% increase in arm circumference 15/10cm proximal/distal to the olecranon on the ipsi- versus contra-lateral arm). Accrual stopped, when 1012 patients had 3-year estimates of lymphedema. Results Between 2015-2021, 2,963 patients were recruited with 2,908 in the intention-to-treat analysis. The 50Gy group comprised 1444 patients(50%) and the 40Gy group 1464 patients(50%). Median age was 57 years (range 23-86). Mastectomy was used in 1383 patients (48%), 1525 patients (52%) had lumpectomy. 953 (33%) patients had sentinel node biopsy (SNB) only (50Gy:32%; 40Gy:33%), and 1946 (67%) had axillary lymph node dissection (ALND) (50Gy:66%; 40Gy:66%); <1% had unknown axillary surgery. SIB was used in 487 (32%) cases with lumpectomy. At median follow-up 4.1 years (IQR 2.9-5.0), the 3-year rates of lymphedema were 8.5% in the whole group; 9.0%(50Gy) versus 8.3%(40Gy), absolute difference (AD) 1.0%(95%CI -1.4%;3.4%), p=0.41. When estimated cumulatively, the 3-year cumulative incidence of lymphedema was 14.9%(95%CI 12.9;17.1) (50Gy) and 15.3%(13.4;17.5) (40Gy), AD 0.4%(-2.5;3.4), p=0.77. The rates of lymphedema were 3.2% (SNB) versus 11.4% (ALND), AD 8.8%(6.6%;10.9%), p<0.01). Estimated by fractionation: 5-year(SNB) 8.4%(50Gy) versus 13.9%(40Gy), AD 5.5%(-1.0;12.1), p=0.10 and for 5-year(ALND) 28.8%(50Gy) versus 26.2%(40Gy), AD -2.6%(-8.7;3.4),p=0.40 (Figure 1B). There was no difference in shoulder motion by randomisation. The 5-year risk of loco-regional recurrence was 2.8%(95%CI 2.0;3.8) (50Gy) and 2.4%(1.6;3.2) (40Gy), AD -0.4%(- 1.6;0.7), p=0.46. The 5-year risk of distant recurrence was 11.1%(9.4;12.8) (50Gy) and 12.0%(10.3;13.8) (40Gy), AD 0.9%(-1.5;3.3), p=0.46. The 5-year risk of death from all causes was 7.8%(6.4;9.4) (50Gy) and 9.9%(8.4;11.6) (40Gy), AD 2.1%(-0.1;4.3), p=0.06. The 5-year risk of death from breast cancer was 5.1%(4.0;6.4) (50Gy) and 7.2%(5.9;8.7) (40Gy), AD 2.1%(0.3;4.0), p=0.02. Conclusion Moderately hypofractionated loco-regional BC irradiation did not cause more arm lymphedema compared to standard. Hypofractionated RT did not influence risk of loco-regional, distant recurrence or death, although 5-yr risk of BC death was significantly higher.