Abstract (1) PURPOSE The aim of the prospective trial was to determine if cell survival in a breast cancer spheroid model following cytostatic treatment in vitro predicts treatment response in breast cancer patients receiving equivalent neoadjuvant therapy. (2) PATIENTS AND METHODS Three-dimensional spheroids were directly generated from fresh tumor biopsy samples of 78 patients eligible for neoadjuvant therapy. Cell survival in vitro, as well baseline clinical and pathological characteristics were correlated with the outcome following treatment of each patient to determine the factor(s) most highly associated with pathological complete response (pCR i.e.ypT0/ypN0) at surgery. (3) RESULTS Cell survival after treatment in the breast cancer spheroid model proved to be a sensitive and specific predictor for pCR in individual breast cancer patients. A mean cell survival of 21.8% was found in the breast cancer spheroid model for 22 patients with pCR versus 63.8% in 56 patients without pCR (P = .001). A receiver operator characteristic analysis determined an area under the ROC curve of 0.86 (95% CI: 0.77 to 0.96) for cell survival compared to classic factors i.e. negative hormone receptor and positive Her2/neu status, and age ≤ 50 years at primary diagnosis (AUC = 0.80, 95% CI: 0.70 to 0.90). A cutoff of 35% cell survival was proposed, which grouped patients according to likelihood for pCR. Out of the 32 patients with values below this threshold, 21 patients (65.6%) and one patient (2.2%) with a cell survival greater than 35% achieved pCR respectively; (sensitivity 95.5% (95% CI: 0.86 to 1.00); specificity 80.4% (95% CI: 0.70 to 0.91)).The specificity was improved to 81% (95% CI: 58 to 95%) if the patient was treated per-protocol. A positive correlation was also found between cell survival in vitro and residual tumor size (P = .021), indicating the possibility of the model to predict degree of response. (4) CONCLUSION The breast cancer spheroid model is a valuable predictor for treatment outcome in patients undergoing neoadjuvant chemotherapy for primary breast cancer. Preclinical selection of the most efficient drugs is a prerequisite to improve pCR. Citation Format: Kathrin ML Halfter, Nina Ditsch, Hans-Christian Kolberg, Holger Fischer, Tanja Hauzenberger, Franz Edler von Koch, Ingo Bauerfeind, Gunter von Minckwitz, Ilona Funke, Alexander Crispin, Barbara Mayer. Prospective cohort study using the breast cancer spheroid model as a predictor for response to neoadjuvant therapy – The SpheroNEO study abstract. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-06-13.