[CLINICAL USEFULNESS OF BIOMARKERS FOR BREAST CANCER]

The estrogen receptor (ER), progesterone receptor (PgR), human epidermal growth factor receptor type 2 (HER2), and Ki67 are biomarkers for early breast cancer. These markers are usually examined by immunohistochemistry (IHC), and positive is defined as more than 1％ for ER or PgR, and a score of 3+ or 2+ with in situ hybridization positivity for HER2. Indications for endocrine therapy and anti-HER2 therapy are determined according to these cutoff values. These markers are also clinically useful for classifying IHC-based subtypes. Although a cutoff value for Ki67 has yet to be determined, ER-positive/HER2-negative breast cancer is further divided into luminal A or B using Ki67 and PgR expression levels. In addition, multigene assays are clinically available to assess the indications for chemotherapy. Since a discordance in biomarkers between primary and metastatic cancer occurs in some cases, rebiopsy is recommended. It is also important to take measures to ensure the accuracy of IHC procedures because the results can easily be affected by a number of factors. The appropriate treatment should be selected by taking the clinical significance of these biomarkers into consideration.