Research Information System
ISTANBUL MEDICAL JOURNAL, vol.27, no.1, pp.55-63, 2026 (ESCI, TRDizin)
Introduction: Microsatellite instability (MSI) is a well-established biomarker in certain malignancies; however, its prognostic and predictive role in breast cancer remains unclear. This study aimed to compare invasive breast carcinoma cases with MSI and those with microsatellite stability (MSS) regarding response to neoadjuvant chemotherapy (NACT), estrogen and progesterone receptor status, c-erbB-2 (CerbB2) expression, Ki-67 proliferation index, and clinicopathological features, using immunohistochemical (IHC) assessment of mismatch repair (MMR) protein expression. Methods: Eighty-seven patients with invasive breast carcinoma who had received NACT were retrospectively analyzed. MMR protein expression was evaluated by IHC for MutL homolog 1 (MLH1), MutS homolog 2 (MSH2), MutS homolog 6 (MSH6), and postmeiotic segregation increased 2 (PMS2). Clinicopathological and IHC variables, including Miller-Payne classification, Pinder lymph node response, hormone receptor status, Ki-67, and CerbB2 expression, were compared between MSI and MSS groups. Survival analyses were performed using the Kaplan-Meier method. Results: MSI was detected in 8% of patients (7/87). Loss of nuclear expression was observed in MLH1/PMS2 (3 cases, 42%), MSH2/MSH6 (2 cases, 29%), and isolated PMS2 (2 cases, 29%). No significant differences were found between the MSI and MSS groups in terms of chemotherapy response, clinicopathological variables, or overall survival and disease-free survival (p>0.05). Conclusion: MSI was identified in 8% of invasive breast carcinoma cases but showed no significant association with NACT response, clinicopathological features, or survival outcomes. Its prognostic and predictive role in breast cancer remains uncertain and warrants confirmation in larger prospective studies.