304P - Impact of estrogen receptor positivity for adjuvant endocrine therapy in luminal T1a/bN0M0 breast cancer: A multi-institutional retrospective observational study

Background

The prognosis for patients with luminal T1a/bN0M0 breast cancer is favorable, and the effectiveness of adjuvant endocrine therapy (ET) is statistically significant but numerically small. The expression level of estrogen receptor (ER) may be associated with sensitivity to ET. This study investgated the therapeutic impact of ER positivity for adjuvant ET in luminal T1a/bN0M0 breast cancer.

Methods

We retrospectively evaluated the effectiveness of adjuvant ET in patients with ER-positive and HER2-negative T1a/bN0M0 breast cancer who underwent surgery between 2008 and 2012 according to ER positivity from 42 institutes. ER expression level was devided into ER-low positive (1%–9% or Allred score 3–4) and ER-high positive (10%–100% or Allred score 5–8). Distant disease-free survival (DDFS) was evaluated, and hazard ratio was adjusted by risk factors, such as menopausal status, nucleat grade and lymphatic invasion.

Results

Among 3907 eligible patients (98 ER-low positive [2.5%] and 3809 ER-high positive [97.5%]), 1692 (43.3) were premenopausal, 253 (6.5%) had high nuclear grade tumors, and 364 (9.3%) had lymphatic invasion. Adjuvant ET was administered to 3305 (84.6%) patients. The median follow-up period was 9.2 years. The 10-year DDFS was 91.0% with ET and 89.5% without ET in patients with ER-low positive tumors (adjusted hazard ratio [HR], 0.87; 95% CI, 0.16–4.86), and 96.0% and 91.2% in those with ER-high positive tumors (adjusted HR, 0.38; 95% CI, 0.26–0.54), respectively. Contralateral breast cancer was less common in patients with ET than those without ET, regardless ER expression (10-year cumulative incidence 0% and 8.6% in ER-low, and 2.0% and 5.2% in ER-high, respectively).

Conclusions

Adjuvant ET is not supported for preventing distant metastasis in patiets with ER-low positive small breast cancer. However, ET may be considered for risk reduction of contralateral breast cancer.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

Ministry of Health, Labour and Welfare and Japan Agency for Medical Research and Development.

Disclosure

All authors have declared no conflicts of interest.