Breast cancer with biomarker reversal during the course of treatment: a case report.

During breast cancer recurrence, drug therapy is planned based on the biological characteristics of the primary tumor. However, the mechanisms underlying these changes have not yet been clarified. A 59-year-old woman underwent breast cancer surgery 23 years previously and received postoperative hormone therapy for 2 years. She had abdominal distention and ascites effusion and was diagnosed with carcinomatous peritonitis due to luminal-type breast cancer after ascites puncture. She received up to the fourth line of treatment. Subsequently, pleural effusion was observed and human epidermal growth factor receptor 2 type breast cancer was diagnosed because of pleurodesis. This case suggests that the cell block diagnostic method based on thoracic and ascites fluid cytology is useful not only for confirming the primary tumor but also for diagnosing the biological characteristics of breast cancer. In the treatment of breast cancer recurrence, it is important to plan the treatment, including aggressive re-biopsy of metastases.