Molecular Subtyping of Breast Cancer by Immunohistochemistry, and Their Association with Histological Grade: A Large Tissue Microarray-Based Study in a Libyan Cohort

AbstractBackground: Breast cancer is the most common malignancy among women worldwide and a leading cause of cancer-related death. Its burden is rising across Africa, including Libya. In resource-limited settings, immunohistochemistry (IHC)-based assessment of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), and Ki-67 provides a practical surrogate for gene-expression-based molecular profiling, and tissue microarrays (TMAs) enable efficient, high-throughput biomarker evaluation. However, data on the distribution of IHC-based molecular subtypes of breast cancer in Libya remain limited. Aim: The present study aimed to characterize the distribution of immunohistochemistry-based surrogate molecular subtypes of breast cancer in a large Libyan cohort, investigate their associations with clinicopathological parameters, and evaluate the utility of tissue microarray-based immunohistochemistry as a practical platform for molecular stratification in a resource-limited setting. Materials & Methods: This retrospective cross-sectional study included 1,073 archival breast cancer specimens evaluated by IHC for ER, PR, HER2, and Ki-67 using whole-tissue sections and TMA blocks. Tumors were classified into four IHC-based surrogate molecular subtypes: Luminal A, Luminal B, HER2-enriched, and triple-negative, using a Ki-67 cut-off of 14%. Associations between molecular subtypes and clinicopathological variables (age, sex, tumor grade, and receptor status) were assessed using chi-square/Fisher's exact tests, independent-samples t-tests, one-way ANOVA, Pearson correlation, and binary logistic regression, with significance set at p < 0.05. Results: The cohort was predominantly female (98.5%; n = 1,057), with a mean age of 52.8 ± 12.0 years and a peak frequency in the 45–59-year group (45.7%). Among 998 tumors with complete biomarker data, Luminal B was the most prevalent subtype (44.0%), followed by Luminal A (21.0%), HER2-enriched (20.8%), and triple-negative breast cancer (14.1%). Molecular subtype was significantly associated with tumor grade and with hormone receptor/HER2 status (all p < 0.001). Age showed a weak inverse correlation with Ki-67 (r = −0.099, p = 0.002), and ER-negative tumors showed significantly higher proliferative activity than ER-positive tumors (mean Ki-67 61.8% vs. 32.5%, p < 0.001). Multivariable logistic regression identified PR positivity, HER2 status, tumor grade, age, and sex as independent predictors of ER status (Nagelkerke R² = 0.721; overall accuracy 89.3%).Conclusion: The predominance of Luminal B tumors and their significant associations with clinicopathological characteristics indicate a comparatively more proliferative breast cancer profile in this Libyan cohort. Combined whole-section and TMA-based IHC proved feasible for large-scale molecular stratification in this resource-limited setting. These findings provide population-specific evidence to support prognostic stratification, personalized treatment planning, and future breast cancer research in Libya and comparable settings.