Objective: When metastasis develops in some breast cancer patients, hormone receptors (HR) and Human Epidermal Growth Factor-2 (Her-2) status can change and the tumor alters its character. We tried to determine the rate of these changes in tumor biology in 110 patients that we followed in our clinic and performed the change of the biopsy from the metastatic site (re-bx). We aimed to determine the biological changes of tumors and, contribute to the literature by examining the relationship of

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... e changes with the adjuvant endocrine treatments (ET) or chemotherapy type (CT). Material and Methods: We included 110 metastatic breast cancer patients in our study. These patients had previously completed their local treatments followed by CT, and those with positive HR completed ET. After the first metastasis developed in the patients, we performed metastasectomy or biopsy from the metastatic site. Results: The median ki-67 value was 25% at the time of primary diagnosis and 30% in re-bx. 20.9% of patients estrogen receptor (ER), 31.8% of patients progesterone receptor (PR) and 26.3% of patients Her-2 changed when metastasis developed. Conclusions: We found that the metastatic tumor has more aggressive properties than the primary tumor. Adjuvant chemotherapy and endocrine treatments or the location of metastasis did not make a significant difference in tumor biology.