Duration and effect of neoadjuvant endocrine therapy on invasive tumor cellularity in hormone receptor-positive breast cancer.

Purpose: Neoadjuvant chemotherapy has been used to evaluate tumor response and downstage hormone sensitive localized breast cancers. However, complete pathological responses are uncommon. Neoadjuvant endocrine therapy (NET) has been used sparingly for the treatment of hormone receptor (HR)-positive breast cancers and frequently for six months or less. There is no clear definition of response to NET nor well-defined parameters for duration of treatment.

Methods: A retrospective chart review of patients with HR-positive localized invasive breast cancers treated with NET for at least 2 months was performed at a single institution. Clinical features, drug selection, duration of therapy, type of surgery as well as pathological characteristics, and residual tumor cellularity were analyzed. A multivariable linear regression model was used to examine the association between NET length and residual invasive tumor cellularity.

Results: 104 evaluable HR-positive invasive breast cancers were treated with NET over an 11-year period. Median age was 69 (range: 31-89) and 88% were post-menopausal. The median duration of treatment was 8 months, with 61% having at least 7 months. Patients receiving NET for 7 months or longer had a significantly lower residual cellularity (20.4%) compared to those treated 2-6 months (34.9%) (p = 0.006). Both pre-treatment and post-treatment Ki-67 ≤ 10% were associated with a lower residual invasive tumor cellularity. Residual invasive cellularity was not associated with menopausal status or NET agent.

Conclusion: Longer NET duration is associated with greater tumor response. Mean residual invasive tumor cellularity was 42% lower among patients receiving 7 or more months of NET.