PAM50- and immunohistochemistry-based subtypes of breast cancer and their relationship with breast cancer mortality in a population-based study.

Purpose: We evaluated the prognostic ability of immunohistochemistry (IHC)-based vs. PAM50-based subtypes for breast cancer mortality in a population-based study of breast cancer. Methods: We included a total of 463 breast cancer cases from the population-based Long Island Breast Cancer Study Project (LIBCSP). IHC-based markers were abstracted from the medical records, while the PAM50-based intrinsic subtypes were assessed from tumor tissues using NanoString nCounter^® Analysis System. Cox proportional hazards models were used to estimate hazards ratios (HRs) for breast cancer-specific mortality associated with subtypes. Results: For IHC-based hormone receptor-positive (HR+) tumors (n = 361), 68.7% were classified as luminal subtypes by PAM50; for HR− tumors (n = 102), 95.1% were classified as non-luminal subtypes. Compared to HR+/HER2− subtype, HR− patients had significantly higher breast cancer mortality (HR−/HER2+: HR = 2.84, 95% CI = 1.58–5.11; triple-negative breast cancer: HR = 2.42, 95% CI = 1.44–4.06). Compared to luminal A, a higher mortality rate was observed for all other PAM50-based subtypes: luminal B (HR = 4.03, 95% CI = 1.97–8.22), HER2-enriched (HR = 6.82, 95% CI = 3.29–14.14) and basal-like (HR = 4.71, 95% CI = 2.24–9.93). Additional subtyping of HR+ patients by PAM50 provided future risk stratification where luminal B patients in this group had significant higher mortality than luminal A patients (HR = 3.93, 95% CI = 1.92–8.03). Similar results were also observed among 291 HR+/HER2− patients, but not among the HR− patients. Conclusions: Our study suggests that for HR+ patients, especially HR+/HER2− patients, additional PAM50-based subtyping would provide better prognostic stratification and improve disease management. [ABSTRACT FROM AUTHOR]