Tumor Ki67 after a 2-week aromatase inhibitor run-in in patients with ER+ early breast cancer proposed for up-front surgery

Background: Endocrine therapy (ET) is effective for endocrine receptor-positive (ER+) early breast cancer (EBC).Ki67 is used as a biomarker of response and prognosis.The POETIC trial showed that short pre-operative ET reduces Ki67, and low levels at 2 weeks predict lower recurrence risk.Objective: This study aims to determine the proportion of patients with Ki67 < 10% in surgical specimens after short-term ET and validate these results in the Portuguese population.Methods: This was two-center, prospective, and single-arm study.Postmenopausal women with ER+, resectable EBC 1 cm received aromatase inhibitors (POAI) for 14 days before surgery.Ki67 was assessed at baseline (Ki67B) and post-surgery (Ki67surg).Results: Twenty-three patients were included, median age 67 (50-84), most ECOG PS 0 (82.6%).All had node-negative (cN0) cT1 -2 tumors.Median POAI duration: 15 days (14-28).Breast-conserving surgery was performed in 73.9%, sentinel node biopsy in 82.6%, and axillary dissection in 17.4%.Pathologically, 60.9% were pN0 and 39.1% pN1 -3.Eight patients (34.8%) received adjuvant chemotherapy.At diagnosis, 95.7% had Ki67 10%, with median Ki67B of 25% (2-70).After POAI, Ki67surg < 10% was found in 44%.Ki67 dropped in 91.3% of patients, with a median difference of 13%.Conclusion: Short-course pre-operative ET induced dynamic reductions in Ki67.Ki67 variation may offer a simple, low-cost method to assess treatment response in ER+ EBC.