Objectives: To evaluate patterns of care in axillary treatment of Dutch patients - who were diagnosed with stage I-IV primary invasive breast cancer, underwent amputation and were found positive by sentinel lymph node biopsy (SLNB) - in the years following the publication of the American College for Surgeons Oncology Group (ACOSOG)-Z0011 and After Mapping of the Axilla: Radiotherapy Or Surgery (AMAROS) trials. Methods: Data from females diagnosed between January 1st 2009 and December 31st 2018 were obtained from the Netherlands Cancer Registry (NKR). Trends in completion axillary lymph node dissection (cALND), administration of adjunctive radiotherapy and, inversely, of omission of any kind of axillary treatment, were revealed by descriptive analyses on SNL positive patients. Results: Of 28,070 cT1-4 breast cancer patients undergoing amputation with SLNB from 2009 up until 2018, 10,706 were diagnosed as SLN positive. In the latter, the frequency of subsequent cALND went down significantly from 78% in 2009 to 10% in 2018 (P < 0.001), while adjunctive radiotherapy increased from 4% to 49% (P < 0.001). Omission of any additional local treatment increased from 18% to 41% (P < 0.001). In N1 patients cALND decreased from 57% to 13% (P < 0.001) and was mostly replaced by radiotherapy, increasing from 2% to 70% (P < 0.001). In N1mi and N0i+ patients, cALND decreased to less than 1% of cases in 2018, while radiotherapy increased from 4% to 40% and 8% to 13% (P < 0.001), respectively. Conclusion: In SLN positive breast cancer patients undergoing breast amputation, cALND had become rare 10 years following the AMAROS and Z0011 trial results. While radiotherapy mostly replaced cALND in the vast majority of N1 patients, less than half of N1mi and only a tenth of N0i+ patients received radiotherapy as the only adjunctive axillary treatment by the end of 2018. Citation Format: Eline EF Verreck, Julia van Steenhoven, Anne Kuijer, Marissa van Maaren, Sabine Siesling, Thijs van Dalen. Trends of axillary treatment in sentinel node-positive breast cancer patients undergoing amputation [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-20-05.

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