Impact of axillary disease extent defined by baseline ¹⁸F-FDG PET/CT on the accuracy of axillary surgical staging after neoadjuvant systemic therapy in clinically node-positive breast cancer

Background: In clinically node-positive patients, sentinel lymph node biopsy (SLNB), marking axillary lymph node with radioactive iodine seed (MARI), and combined SLNB/MARI (RISAS-procedure) can replace axillary lymph node dissection (ALND) after neoadjuvant systemic therapy. Surgical staging outcome can be combined with baseline axillary disease on ¹⁸F-FDG PET/CT. This study assessed whether baseline axillary disease on ¹⁸F-FDG PET/CT affects the accuracy of staging-procedures. Second, when staging-procedures detected residual disease, it was assessed whether baseline axillary disease on ¹⁸F-FDG PET/CT affected the probability of remaining positive nodes at completion ALND (cALND). Method: Included were patients with baseline ¹⁸F-FDG PET/CT within the RISAStrial (NCT02800317). Patients underwent the RISAS-procedure followed by cALND. False negative rates were stratified by limited or advanced baseline axillary disease (1-3 vs. ≥4 hypermetabolic lymph nodes). When staging-procedures detected residual disease, the probability of remaining positive nodes at cALND was stratified by baseline axillary disease. Results: Of 185 patients, 116 had limited and 69 had advanced baseline axillary disease. Staging-procedures had higher accuracy in limited than advanced baseline axillary disease. When the RISAS-procedure detected residual disease, the probability of remaining positive nodes at cALND was lower in limited than advanced baseline axillary disease (44.9% vs. 91.5%,p < .001). When SLNB or MARI detected residual disease, the probability of remaining positive nodes at cALND was >88.4%, irrespective of baseline axillary disease. Conclusion: Staging-procedures had higher accuracy in patients with limited than advanced axillary disease on baseline ¹⁸F-FDG PET/CT. When staging-procedures detected residual disease, the probability of remaining positive nodes at cALND remained high.