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Background: Neoadjuvant endocrine therapy (NET) is an approach that allows real-time evaluation of drug efficacy as well as investigation of the biological and molecular changes that occur after estrogenic deprivation. This evaluation is mainly performed in surgical specimens after NET and it has been used as a research tool to obtain prognostic and predictive information using tumour response to decide adjuvant treatment. In this setting, there are not many validated biomarkers to predict response beyond Ki67 expression and Preoperative Prognostic Index (PEPI score) in NET. The aim of this study is to determine if the tumour cellularity size (TCS) in surgical specimen after NET correlates with PEPI score and Ki67 expression. Methods: Retrospective study of postmenopausal patients with estrogen receptor (ER) positive/HER2 negative resectable breast cancer, treated with an aromatase inhibitor for at least 4 months prior to surgery. Pathological characterization of tumour specimens: Evaluation of the percentage of residual tumour cellularity of formalin fixed paraffin embedded surgical specimens and immunohistochemistry characterization of ER and Ki67. Tumour cellularity size: calculated by combining the percentage of residual tumour cellularity and tumour pathological size. Results: N=104. Tumour characteristics at surgery and breakdown for the calculated PEPI score: table 1. Correlation between the percentage of Ki67 positive cells at surgery and TCS: (r=0.2503) p=0.04 (95% CI, 0.0014 to 0.4700). Correlation between TCS and PEPI score: (r=0.2582) p=0.05 (95% CI, -0.0131 to 0.4940). Conclusions: Tumour cellularity size is a promising biomarker to determine response and prognosis after NET. There is a need to find other biomarkers to predict response after NET. Table 1Pathology/Biomarker statusPEPI score RFS pointsNo. of patientspTNot available (NA)1pT1/T20102pT3/T431pNNA7Negative075Positive322KI67 levelNA10%-2.7%039>2.7%-7.3%131>7.3%-19.7%119>19.7%-53.1%213>53.1%31ER-status Allred scoreNA120-2303-8092PEPI scoreNA190281342533495561PEPI groupNot calculated19I (0 score)28II (1-3 score)42III (≥4 score)15 Citation Format: Joanna Ines Lopez Velazco, María Otaño, Lide Larburu, Ainhara Lahuerta, Kepa Elorriaga, Virginia Segur, Juan Carlos Irizabal, Ana Martínez, Lourdes Jáuregui, Maria M. Caffarel, Ander Urruticoechea. Tumour cellularity size as a biomarker to predict response after neoadjuvant endocrine therapy: Correlation analysis between Ki67 expression and PEPI score [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-39.