ESTRO 2021 Abstract Book

S42

ESTRO 2021

Conclusion The LRRT SG seems to result in a satisfactory oncological outcome, with low 5-year LRR rates. LRR rates did not differ between risk groups, while especially high-risk patients had a worse RFi and OS. Since the risk groups were based only on ypN-status, further analyses will be performed to investigate the influence of RT volumes, type of surgery, and tumour characteristics on oncological outcome.

OC-0068 Elective internal mammary node irradiation in women with node-positive breast cancer

Abstract withdrawn

OC-0069 Quality assessment of locoregional breast cancer radiotherapy in the international SENOMAC- trial S. Alkner 1 , D. Lundstedt 2 , E. Wieslander 1 , M. Berg 3 , I. Kristensen 1 , J. Ahlgren 4 , B. Offersen 5 , J. de Boniface 6 , H. Dahl Nissen 7 , O.B.O.T.S.T.G. . 8 1 Lund University, Skane University Hospital, Department of Oncology and Pathology, Skane Clinic of Oncology, Lund, Sweden; 2 University of Gothenburg, Sahlgrenska University Hospital, Department of Oncology, Gothenburg, Sweden; 3 Lillebaelt Hospital, Department of Medical Physics, Vejle, Denmark; 4 Uppsala University Hospital, Regional Cancer Center Uppsala Orebro, Uppsala, Sweden; 5 Aarhus University, Aarhus University Hospital , Department of Experimental Clinical Oncology, Deparment of Oncology, Aarhus, Denmark; 6 Karolinska Institutet, Capio St. Göran's Hospital, Department of Molecular Mediciene and Surgery, Department of Surgery. , Stockholm, Sweden; 7 Lillebaelt Hospital, Department of Medical Physics., Vejle, Denmark; 8 The SENOMAC Trialists' Group, The SENOMAC Trialists' Group, Stockholm , Sweden Purpose or Objective The SENOMAC trial randomizes clinically node-negative breast cancer patients (pts) with macrometastases (>2mm) in up to two sentinel nodes to axillary lymph node dissection (ALND) versus surveillance (sentinel lymph node biopsy only) with the main endpoint overall survival. Independent of group assignment, pts receive locoregional RT according to national guidelines. The inclusion of axillary level I in the clinical target volume (CTV) differs between countries and sites. In Denmark, level I is within the target if <10 lymph nodes are removed (i.e. if ALND is not performed). In Sweden, the decision to include level I has been site specific. Since the delivered axillary radiotherapy affects outcome, we here evaluate targets and dose distribution from the initial phase of the trial using treatment plans from Sweden (SE) and Denmark (DK) (together enrolling 94% of patients) with a special focus on the inclusion of axillary level I. Materials and Methods Treatment plans from pts included before June 2019 were reviewed: Swedish RT plans were available in 834 out of 919 pts (91%), and Danish RT plans for 302 out of 332 pts (91%). The axillary CTV was dichotomized as