151P Exploring the predictive potential of 18FFDG 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.