Discrimination between Complete versus Non-Complete Pathologic Response to Neoadjuvant Therapy Using Ultrasensitive Mutation Analysis: A Proof-of-Concept Study in BRCA1 -Driven Breast Cancer Patients

Neoadjuvant chemotherapy (NACT) for breast cancer (BC) often results in pathologic complete response (pCR), i.e., the complete elimination of visible cancer cells. It is unclear whether the use of ultrasensitive genetic methods may still detect residual BC cells in complete responders. Breast carcinomas arising in mutation carriers almost always carry alterations of the gene thus providing an opportunity to address this question. The analysis of consecutive BC patients treated by NACT revealed a higher pCR rate in -driven vs. -wildtype BCs (13/24 (54%) vs. 29/192 (15%), < 0.0001). Twelve pre-/post-NACT tissue pairs obtained from mutation carriers were available for the study. While mutation was identified in all chemonaive tumors, droplet digital PCR (ddPCR) analysis of the post-NACT tumor bed revealed the persistence of this alteration in all seven pCR-non-responders but in none of five pCR responders. Eleven patients provided to the study post-NACT tissue samples only; next-generation sequencing (NGS) analysis revealed mutated copies in all six cases without pCR but in none of five instances of pCR. In total, mutation was present in post-NACT tissues in all 13 cases without pCR, but in none of 10 patients with pCR ( < 0.000001). Therefore, the lack of visible tumor cells in the post-NACT tumor bed is indeed a reliable indicator of the complete elimination of transformed clones. Failure of ultrasensitive methods to identify patients with minimal residual disease among pCR responders suggests that the result of NACT is a categorical rather than continuous variable, where some patients are destined to be cured while others ultimately fail to experience tumor eradication.