The goal of this observational study is to examine whether markers of PI3K pathway activation are associated with endocrine therapy response and clinicopathologic features in estrogen receptor-positive, HER2-negative breast cancer. The main questions it aims to answer are:

Are immunohistochemical levels of phosphorylated AKT (p-AKT) and phosphorylated S6 (p-S6) different between endocrine-sensitive and endocrine-resistant breast cancer cases? Do different levels of p-AKT and p-S6 show distinct clinicopathologic and histologic characteristics, including features of the tumor microenvironment?

Archived tumor tissue from patients who received adjuvant endocrine therapy as part of routine clinical care will be analyzed. Biomarker expression will be correlated with clinicopathologic parameters, including tumor-infiltrating lymphocyte density, tumor budding, and other histologic features, to explore associations with tumor behavior and outcomes.

Endocrine therapy is the cornerstone of treatment for estrogen receptor-positive breast cancer; however, a substantial proportion of patients develop resistance during the disease course. Activation of the PI3K/AKT/mTOR signaling pathway has been implicated as a key mechanism underlying resistance to endocrine therapy and disease progression. While genomic alterations in PI3K pathway-related genes are commonly assessed, they do not consistently reflect pathway activation or predict therapeutic response, highlighting the need for practical surrogate biomarkers.

The aim of this study is to evaluate immunohistochemical markers of PI3K pathway activation in estrogen receptor-positive, HER2-negative invasive breast carcinoma and to explore their associations with endocrine therapy response and clinicopathologic characteristics. Specifically, the study examines whether levels of phosphorylated AKT (p-AKT) and phosphorylated S6 (p-S6) differ between endocrine-sensitive and endocrine-resistant cases and whether these markers correlate with histologic features related to tumor biology and the tumor microenvironment.

This is a retrospective observational study including patients with estrogen receptor-positive, HER2-negative invasive breast carcinoma who received adjuvant endocrine therapy as part of routine clinical care. Archived formalin-fixed, paraffin-embedded (FFPE) tissue blocks from excisional surgical specimens will be retrieved, and clinicopathologic and follow-up data will be collected from medical records.

Cases will be classified into endocrine-sensitive and endocrine-resistant groups based on international consensus definitions using the timing of disease recurrence relative to endocrine therapy. Immunohistochemical staining for p-AKT and p-S6 will be performed on FFPE tissue sections and evaluated using a semi-quantitative scoring system. Histopathologic assessment will include standard reporting parameters as well as additional features such as tumor-infiltrating lymphocyte density, tumor budding, lymphovascular invasion, perineural invasion, and tumoral necrosis. Associations between biomarker expression, clinicopathologic features, and follow-up outcomes will be explored.

Inclusion Criteria:

• Patients diagnosed with invasive breast carcinoma.
• ER-positive and HER2-negative tumor status.
• Received adjuvant endocrine therapy as part of routine clinical care.
• Archived formalin-fixed paraffin-embedded (FFPE) tumor tissue available from excisional specimens (mastectomy).
• Available medical records documenting treatment history and follow-up outcomes.

Exclusion Criteria:

• Cases with missing or unavailable FFPE tissue blocks.
• Cases with insufficient tissue quality for immunohistochemistry (e.g., severely degraded or exhausted FFPE block).
• Cases with unknown or incomplete follow-up or treatment data.

Patients with estrogen receptor-positive (ER+), HER2-negative invasive breast cancer who received adjuvant endocrine therapy and have available formalin-fixed, paraffin-embedded (FFPE) tumor tissue archived in the Surgical Pathology Laboratory of Assiut University Hospital, as well as corresponding clinical follow-up data in the medical records of the Oncology Department at Assiut University Hospital.

Individual participant-level data will not be publicly shared because the study uses archived clinical and pathology records containing sensitive patient information. Although the data will be de-identified, sharing it broadly could risk patient privacy and confidentiality. Data may be shared on a case-by-case basis with qualified researchers after institutional review and approval.