[18F]FDG PET/MRI for axillary staging in node positive breast cancer patients receiving primary systemic therapy. A prospective single arm trial.

Management of axillary nodes in breast cancer (BC) remains controversial. Increasingly, node-positive patients receive primary systemic therapy (PST), achieving axillary downstaging in up to 70% of cases. However, current imaging methods, despite being pivotal in the choice for PST, show limited accuracy in assessing treatment response. This study evaluated the diagnostic performance of hybrid [18F]FDG PET/MRI as non-invasive, single-step tool for post-PST nodal assessment. This prospective, single-arm, monocentric interventional trial (SNBvsPET/MRI1, ClinicalTrials.gov NCT04826211) was conducted at San Raffaele Scientific Institute, Milan. Patients with node-positive BC eligible for PST were enrolled between September 2019 and December 2024. In addition to routine imaging, including axillary ultrasound (A-US), [18F]FDG PET/MRI was performed before and after PST. Scans were independently reviewed by two radiologists and two nuclear medicine physicians. Final pathology served as the reference standard. Of 109 screened patients, 68 were included in the final analysis Mean(SD) age was 52.3(11.0) years. Residual axillary disease was pathologically confirmed in 30(44.2%) patients. [18F]FDG PET/MRI correctly identified 16 cases (sensitivity 53.3%, specificity 68.4%, PPV 57.1%, NPV 65.0%). Diagnostic accuracy varied by BC subtype with higher PPV in luminal (85.7%) and greater NPV in HER2+ and triple-negative tumors (83.3%). Overall, the performance of [18F]FDG PET/MRI and A-US were not significantly different (p > 0.05). [18F]FDG PET/MRI accuracy for axillary staging in node-positive BC patients following PST is limited. Our findings are hypothesis-generating to design future trials and suggest that tailoring imaging to molecular subtype may optimize axillary evaluation and better inform surgical decision-making after PST.