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Purpose: To evaluate the value of early FDG-PET (18F-Fluorodeoxyglucose-Positron Emission Tomography) metabolic criteria for prediction of pathologic complete response in axilla (pCRAx) after neoadjuvant chemotherapy (NAC) in breast cancer. Methods: Inclusion criteria were all T-stage breast cancers, non-metastatic, with initial lymph node involvement estimated by PET +/- lymph node biopsy, treated with NAC followed by surgery with axillary lymph node dissection (ALND), managed at the George-François Leclerc Cancer Center in Dijon, France, between 2009 and 2019. A PET was performed before and after the first course of chemotherapy (PET1 and PET2). pCRAx was defined as the absence of invasive cells in the nodes at the time of ALND (i.e. ypN0). The Sataloff classification was used as reference on each pathological report. Patients with a Sataloff NA classification (i.e. evidence of therapeutic effect, and no residual disease) and, if axillary involvement was proven at diagnosis, NB (i.e. no metastasis, no therapeutic effect) were considered as pCRAx. The PET metabolic criteria studied in the axilla were: - SUVmax (Standard Uptake Value) on PET1 and PET2 = fixation in the axillary voxel with the highest activity (kBq/mL)/(injected dose (kBq)/weight (g)) - ΔSUVmax (%) = metabolic response after the first course of NAC = 100 x (SUVmax1 - SUVmax2)/(SUVmax1). Univariate and multivariate analysis were performed to identify factors (clinical, pathologic, metabolic) that may be associated with pCRAx. Relationships between baseline TEP uptake and prognostic parameters were assessed using Receiver Operating Characteristic (ROC) curves. Results: Among 188 patients included, the rate of pathologically proven node involvement was 63.3% (n=119). The pCRAx rate was 45.7% (n=86/188) but varied according to tumor subtypes: 14.5% (n=9/62) of HR(Hormone Receptor)+/HER2-negative, 47.7% (n=21/44) of HR+/HER2-positive, 61.4% (n=27/44) of triple-negative (TN) and 76.3% (n=29/38) of HR-/HER2-positive. Factors significantly associated with pCRAx were by univariate analysis: HER2-positive (HR+ and HR-) and TN subtypes (p< 0.001), SBR (Scarff-Bloom-Richardson) grade (p=0.01), breast pCR (ypT0/is) (p< 0.001), SUVmax2 (p=0.01) and ΔSUVmax (p< 0.001). By multivariate analysis, it persisted the HR-/HER2-positive (p=0.02) and TN (p=0.02) subtypes and breast pCR (p< 0.001). In global population, a decrease in ΔSUVmax of 63% was the optimal threshold to predict pCRAx (Area Under the Curve AUC = 0.73) with a sensitivity (Se) of 51% and specificity (Sp) of 83%. ΔSUVmax remains the best performing parameter in TN (AUC = 0.72; Se at 52%; Sp at 88%). In HR-/HER2-positive patients, SUVmax2 appeared to be a better predictor of pCRAx than ΔSUVmax. A SUVmax2 value of 1.99 was the optimal threshold for predicting pCRAx (AUC = 0.72), yielding a Se of 66% and a Sp of 78%. None of the PET criteria predicted axillary response with sufficient accuracy for HR+ subtypes. Conclusion: PET alone does not appear to be sufficient to predict pCRAx. It seems necessary to use other parameters, whether clinical, biological or imaging, to discriminate responders from non-responders to NAC in order to adapt the subsequent surgical management. Citation Format: ELOISE MICHEL, FRANCOISE BELTJENS, ALEXANDRE COCHET, JEAN LOUIS ALBERINI, CHARLES COUTANT, CLEMENTINE JANKOWSKI. Utility of 18F-FDG PET/CT for the prediction of pathologic complete response in axilla to neoadjuvant chemotherapy in breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-01-11.