Change from triple negative type to triple positive type: A case report of discordance of re-biopsy from primary breast cancer

The patient was a 60-year-old woman who had visited a clinic with the chief complaint of a mass in the right breast prior to being referred to our hospital. Breast examination revealed the presence of a 3-cm hard elastic mass in the C region of the right breast. Computed tomography (CT) further indicated metastases to the liver and lungs. Upon needle biopsy of the primary tumor, the patient was diagnosed with triple-negative (ER (-), PgR (-), HER2 (-)) invasive lobular carcinoma. Chemotherapy was successful in achieving a transient partial response (PR); however, the tumor later advanced to a progressive disease (PD) after five cycles of oral fluoropyrimidine derivative therapy (S-1). Re-biopsy of the primary tumor revealed that the tumor was triple-positive (ER (+), PgR (+), HER2 (+)). The patient was subsequently treated with anti-HER2 therapy and has since achieved complete response (CR). Although biological changes sometimes occur from the primary to the metastatic tumor, changes in the primary tumor itself during the course of treatment is a rare event. Furthermore, the transition from triple-negative to triple-positive status is very uncommon. Re-biopsy rarely changes the biological characteristics of a tumor; however, biological changes can have a significant impact on treatment if they do occur. Thus, it is important to perform a re-biopsy if the current treatment results in PD.