The impact of neoadjuvant chemotherapy on immunohistochemistry and molecular subtype in breast cancer: A retrospective analysis from Oman.

[BACKGROUND] Neoadjuvant chemotherapy (NACT) is used in breast cancer (BC) to downsize and downstage the tumor before surgery. Different studies were conducted looking into the alterations in the immunohistochemistry after NACT, which may alter the adjuvant treatment. Our study aimed to assess the changes in immunohistochemistry biomarker status (estrogen receptor [ER], progesterone receptor [PR], and human epidermal growth factor receptor 2 [HER2]) in BC cells after administration of NACT at a single institution in Oman.

[METHODS] We conducted a retrospective cross-sectional study on patients with BC with residual disease post-NACT in a single institution and studied immunohistochemistry status changes before and after NACT. We used the McNemar test to evaluate the receptor changes, and logistic regression to assess the effect of risk factors on receptor status change.

[RESULTS] All biomarkers changed after NACT, with a tendency for BC cells to lose PR and HER2 expression and gain ER expression. The immunohistochemistry changes were 36/114 (31.6%) in PR, 7/114 (6.1%) in ER, and 11/114 (9.6%) in HER2. BC subtypes changed in 4/114 (3.5%) of HER2+ve, 10/114 (8.8%) of hormone positive (ER or PR) with HER2 overexpression (HER2+ve/HR+ve), 3/114 (2.6%) in hormone positive with HER2 negative (HR+ve), and 3/114 (2.6%) triple-negative BCs. These changes resulted in adjuvant treatment adjustments in 9/110 (8.18%) patients with residual disease.

[CONCLUSION] Changes in the expression of all immunohistochemistry biomarkers and BC subtypes occurred after NACT, which led to changes in adjuvant treatment in specific cases.