Introduction: Currently, the indication for neoadjuvant chemotherapy is increasing in the treatment of breast cancer. Variability in the expression of biomarkers following neoadjuvant treatment has been observed, which could be accompanied by changes in the adjuvant treatment.

Objectives: The primary objective was to evaluate the variability of biomarkers prior to and following neoadjuvant therapy. Secondary objectives were to determine which tumour subtype (as determined by immunohistochemical markers) most frequently achieved pathological complete response (pCR); whether the biomarker variation resulted in a change in immunophenotype and subsequently modification to the adjuvant treatment.

Materials and methods: A retrospective observational analysis was carried out on patients with a diagnosis of breast cancer who had neoadjuvant therapy prior to surgery in the Breast Care Service of the Buenos Aires British Hospital between January 2009 and June 2020.

Results: One hundred and seventy-two patients were included. The pCR rate was 28.5%. The tumour immunophenotype that achieved pCR most frequently was the hormone receptor negative /HER2 group with a value of 85.2%. The analysis was carried out on the 123 patients with residual disease. The observed variability for oestrogen receptors (ER) was 8.9%, for progesterone receptors (PR), 29.9% and for HER2, 13.8%. These changes were statistically significant. There were changes to the tumour immunophenotype in 26 cases (21.1%) with modifications to the adjuvant treatment in nine of these (34.6%; 7.3% of all tumours with residual disease).

Conclusions: In this study, we observed statistically significant variability in the expression of ER, PR and HER2 prior to and following neoadjuvant treatment, which identified modifications in the tumour immunophenotype in 21.1%, and changes to the adjuvant treatment in 7.3% of all tumours with residual disease, justifying the re-assay of biomarkers in the surgical specimen.