Prognostic Significance of Progesterone Receptor Expression in Estrogen-Receptor Positive, HER2- Negative, Node-Negative Invasive Breast Cancer With a Low Ki-67 Labeling Index

Abstract Introduction We showed the clinical implications of the Ki-67 labeling index (LI) in hormone receptor (HR)-positive, HER2-negative (HER2 − ) breast cancer patients with lower risk (luminal A [LA]-like) and higher risk (luminal B [LB]-like) subtypes. Patients and Methods Three hundred sixty-nine patients with HR + , HER2 − cancers were eligible and we dichotomized these patients into LA-like and LB-like according to Ki-67 LI (14% cutoff) and analyzed prognostic significance of progesterone receptor (PR) expression. Results Of 205 LA-like and 163 LB-like subtypes, PR was positive in 149 (73%) and 103 (63%). PR expression was a prognostic factor in the LA-like subtype but not in the LB-like subtype. In LA/PR + , LA/PR − , and LB-like subtypes, the 12-year disease-free survival (DFS) rates were 94.8%, 81.6%, and 79.7% ( P  = .03), and breast cancer-specific survival (BCSS) rates were 98.4%, 97.4%, and 92.0%, respectively ( P  = .05). Late recurrence occurred in LA/PR − subtype, and differences in prognosis between LA/PR + and LA/PR − subtypes emerged >5 years after surgery. Twelve-year DFS rates of the LA/PR − subtype were almost equal to those of the LB-like subtype, whereas 12-year BCSS of the LA/PR − subtype was superior to that of the LB-like subtype. In multivariate analysis, PR expression and tumor size were significant or nearly significant prognostic factors. Conclusion PR expression and tumor size were independent prognostic factors in the LA-like subtype, and the LA/PR − subtype had the higher risk of recurrence, especially late recurrence, than the LA/PR + subtype. In LA-like breast cancers, stratification of prognosis according to PR expression and tumor size is important.