151P Exploring the predictive potential of [18F]FDG PET/CT for pathological complete response to neoadjuvant therapy: A prospective study

The excellent prognosis of breast cancer (BC) patients who achieve a pathological complete response (pCR) after neoadjuvant chemotherapy (NAC) has led to a paradigm shift towards omitting surgery in selected patients. Biopsy is currently the only alternative to surgery for evaluating residual cancer after NAC. Our aim was to evaluate the value of [18F] FDG PET/CT in predicting pCR after NAC in patients with BC. We conducted a prospective study of patients with newly diagnosed BC undergoing NAC. All patients underwent [18F] FDG PET/CT before and after NAC, followed by surgery. We collected clinicopathological data. We assessed response to NAC according to final pathology, categorising patients as either pCR or non-pCR. The residual cancer burden (RCB) index was also assessed. We extracted semi-quantitative parameters for the primary tumour on both baseline and preoperative PET/CT. Group differences were analysed using the Kruskal-Wallis test, and multivariate logistic regression was used to explore correlations between outcomes and potential predictors, with statistical significance set at p <0.05. We included 134 patients with BC who were predominantly HER-2+ (n=74) and had stage II disease (n=104). A pCR to NAC was achieved in 69 patients. The RCB index could be assessed in 94/134 patients, with 59 patients classified as RCB-0, 5 as RCB-I, 24 as RCB-II and 6 as RCB-III. Baseline TBRmax correlated with tumour characteristics including BC subtype (9.56 vs. 5.6 vs. 10.2 for HR+, HER-2+ and TNBC; p=0.005) and ki-67 (p=0.002). Baseline SUV did not discriminate NAC responders, while preoperative TBRmax correlated significantly with pCR (1.09 vs. 1.75 for pCR vs. non-pCR; p<0.001) and RCB index (p<0.001). TBRmax emerged as a significant predictor of pCR both at baseline (OR 1.07, p-value 0.04) and at preoperative assessment (OR 0.2, p<0.001). Only preoperative TBRmax was found to be a significant predictor of RCB index (OR 5.5, p=0.004). PET-derived parameters are valuable predictors of pCR in BC patients undergoing NAC. These findings contribute to the evolution of BC treatment strategies and support the incorporation of [18F] FDG PET/CT into decision making for NAC response assessment.