[A Case of Advanced Breast Cancer with Altered Biology by Eribulin Chemotherapy]

In the treatment of advanced breast cancer, onlya few drugs confer overall survival(OS)benefit. Eribulin is a drug that was shown to extend OS in an international phase Ⅲtrial; however, the underlying mechanism is thought to involve cancer microenvironment regulation. The concept of "breast cancer subtype discordance" implies the biological changes that accompany treatment. Herein, we encountered a case of advanced breast cancer in a 54-year-old woman that showed biological changes after eribulin chemotherapy. The patient noticed a lump in her left breast and visited a nearby doctor, who referred her to our hospital for close examination and treatment. Ultrasonographyrevealed a large mass at the center of the left mammarygland and axillaryly mph node swelling. Core-needle biopsyconfirmed an invasive ductal carcinoma(ER stronglypositive, PgRnegative, HER-2 negative, Ki-67 low expression). CT findings showed multiple lung metastases. Letrozole was administered for cT4N2M1, stage Ⅳ, Luminal A, which showed progression to the left side with advances in breast cancer. Six months later, the primarytumor and axillaryly mph nodes showed progression. Subsequent treatment with eribulin was started, and partial response was obtained; however, new lymph node metastasis developed in the axilla after 11 cycles. The primary tumor and axillaryly mph nodes showed stronglypositive ER expression, were PgR-negative and HER2-positive, and showed Ki-67 low expression and HER2-positive conversion.