Impact of Endocrine Therapy, Menstrual Cycle, PAM50, Ki67 on Treatment Decisions in HR+ and HER2- Breast Cancer

This study investigates how hormonal status (including menstrual cycle phase) and a short course of hormone therapy before surgery affect certain biological markers — including a gene test called PAM50 and a protein called Ki67 — in women with early-stage hormone receptor-positive, HER2-negative breast cancer. The goal is to better understand how to personalize treatment decisions. **You may be eligible if...** - You are a woman aged 18 or older - You have been diagnosed with unilateral (one-sided), non-metastatic invasive breast cancer that is estrogen or progesterone receptor-positive and HER2-negative - Your underarm lymph nodes appear clear on examination and ultrasound - You are scheduled for surgery and a short course of pre-operative hormone therapy (Tamoxifen, aromatase inhibitor, or Goserelin) for 2–4 weeks - Your Ki67 result from a core biopsy is available **You may NOT be eligible if...** - You have HER2-positive cancer - You have bilateral breast cancer - You need or are recommended for neoadjuvant chemotherapy - You have had another cancer in the past 10 years - You are pregnant or breastfeeding - You have used hormonal contraception within 6 months before enrollment Talk to your doctor to see if this trial is right for you.