The impact of neoadjuvant systemic therapy on breast conservation rates in patients with HER2-positive breast cancer: Surgical results from a phase II randomized controlled trial.

Background: Neoadjuvant systemic therapy (NST) induces tumor shrinkage and boosts the chance of breast-conserving thearpy (BCT) in patients with breast cancer. However, only a few trials have evaluated the effect of NST in conversion from BCT ineligibility to BCT eligibility in HER2-positive breast cancer.

Methods: We conducted the surgical sub-study of a phase II randomized trial, which compared standard neoadjuvant treatment or an experimental treatment modified according to the interim Ki-67 evaluation in women with stage II or III HER2-positive breast cancer. The treating surgeons assessed eligibility for BCT before and after NST. We evaluated the change in BCT eligibility following NST. We also analyzed the type of surgery performed and the success rate of BCT.

Results: Two hundred six patients were included in this study. Of these, 44.0% were considered BCT candidates at baseline, while 69.8% were deemed eligible for BCT after NST (P < 0.001). Among non-BCT candidates at baseline, 46% successfully converted to BCT candidates. Of 139 patients deemed eligible for BCT following NST, 84.2% attempted BCT, and successful BCT, defined as tumor-free at all surgical margins, was achieved in 96.8% of patients. Different treatment arms did not affect the rate of post-NST BCT eligibility (70.0% vs 69.7%).

Conclusions: This study demonstrated that NST resulted in an absolute increase of 25.8% in the rate of BCT eligibility in HER2-positive breast cancer. About a half of non-BCT candidates converted to BCT candidates. BCT was successful in most patients who attempted BCT. There were still patients who chose mastectomy even though they were deemed eligible for BCT. Patients considered BCT-ineligible due to large tumor size most likely converted to BCT-eligible with NST. On the other hand, NST had less impact on the surgical indication of patients with multicentric disease or probable poor cosmetic outcome.