ESTRO 2022 - Abstract Book

S1005

Abstract book

ESTRO 2022

Conclusion Conservative treatment is a good and safe alternative to mastectomy for patients who want to preserve their breast.

PO-1185 Targeting therapy-resistant metastatic clones with SBRT in oligoprogressive breast cancer

R. Van den Begin 1 , S. Bellal 1 , A. De Caluwé 1 , P. Aftimos 2 , Z. Paquier 3 , G. Gebhart 4 , L. Buisseret 2 , P. Martinive 1 , F. Lefranc 5 , D. Van Gestel 1 1 Jules Bordet Institute, Radiation Oncology, Brussels, Belgium; 2 Jules Bordet Institute, Medical Oncology, Brussels, Belgium; 3 Jules Bordet Institute, Medical Physics, Brussels, Belgium; 4 Jules Bordet Institute, Nuclear Medicine, Brussels, Belgium; 5 Erasmus Hospital, Neurosurgery, Anderlecht, Belgium Purpose or Objective During systemic treatment for metastatic breast cancer (mBCa), disease progression is sometimes limited to a few resistant sites. Treating this oligoprogression with metastasis-directed therapy such as stereotactic body radiotherapy (SBRT) may allow to extend the benefit of the ongoing systemic line. Given the paucity of data in mBCa, we aimed to investigate the outcomes after SBRT for oligoprogressive disease in mBCa. Materials and Methods Forty-one patients with mBCa treated with SBRT between 2016 and 2020 at our department for maximum 5 extracranial oligoprogressive lesions were included in this retrospective study. All progressive lesions must have been treated locally and the ongoing systemic treatment had to be continued after SBRT. Primary endpoint was next systemic treatment-free survival (NEST-FS), other endpoints were progression-free-survival (PFS) and overall survival (OS), all measured from start of (first) SBRT. PFS included all types of progression as an event, also new oligoprogression amenable to new local therapy. Results All patients had HR+ or HER2+ disease. Eighty percent of patients had a grade 2 or 3 invasive ductal carcinoma and 98% of patients demonstrated 1-3 oligoprogressive lesions. Oligoprogression occurred in bone, lymph nodes, lung or intramammary lesions. According to the ESTRO-EORTC definitions, 12 patients (29%) had genuine oligometastatic disease while 29 (71%) showed induced oligoprogression. Median follow-up amounted to 13,7 months. Eleven patients (27%) received repeated local treatments for repeated oligoprogression. Median NEST-FS was 9,2 months (95%CI: 8,0-10,4 months), while median PFS amounted to 5,5 months (4,7-6,3). OS was estimated at 65% (44-80%) at two years. Main significant predictors for superior NEST-FS were low disease burden and having less than four previous lines of systemic treatment. As to the type of treatment, best results were seen for patients receiving HER2-targeted therapy or endocrine agents +/- CDK4/6-inhibitors. Acute and late local grade 3 toxicity occurred in 3% and 4%, respectively, with 0% local grade 4-5 toxicity. Conclusion SBRT in oligoprogressive mBCa appears effective in extending the benefit of systemic treatment, with a median additional 9,2 months before therapy is changed. Trials are warranted to confirm these results prospectively.

PO-1186 Stereotactic radiotherapy to primary breast cancer in de novo stage IV disease: preliminary results