In certain cases, some institutions use fluorodeoxyglucose positron emission tomography combined with CT (FDG-PET-CT) scans to assess response to neo-adjuvant chemotherapy (NAC) in lymph node-positive breast cancer (cN+), to determine the extent of surgery and subsequently radiotherapy. In this study, we assessed the diagnostic accuracy of FDG-PET-CT to determine the axillary response to NAC using the histopathology results as the golden standard.

Between 2016 and 2022, all women with cN+ breast cancer receiving NAC and axillary surgery were retrospectively identified. Patients who underwent pre- and post-NAC staging with FDG-PET-CT were included. Excluded were patients with previous ipsilateral breast cancer, occult breast cancer or bilateral breast cancer. The histopathology lymph node results from surgery were used to calculate the diagnostic accuracy of FDG-PET-CT for detecting axillary lymph node metastases post-NAC.

Seventy-five patients were included. Forty-one patients (55%) had histologically proven axillary metastatic disease at the response evaluation after NAC, although FDG-PET positivity was only apparent in nine of these patients. There was a sensitivity of 22% (95% CI 11–38%) for FDG-PET-CT in detecting axillary lymph node metastases after NAC and a specificity of 94% (95% CI 80–99%). The positive predictive value was 82% (95% CI 48–98%) and the negative predictive value (NPV) was 50% (95% CI 37–63%).

FDG-PET-CT scans have a low sensitivity and NPV to identify residual disease after NAC, indicating that these scans are unlikely to aid in decision making. Omission of post-NAC FDG-PET-CT should be considered in lymph node response evaluation unless other clinical indications with no other alternative.