# Case Report: Comparison different response after neoadjuvant chemotherapy of the breast with cell-in-cell—report of two cases

## TL;DR

This case report compares two HER2-positive breast cancer patients with different responses to neoadjuvant chemotherapy, focusing on changes in cell-in-cell structures.

## Contribution

The report introduces cell-in-cell structures as potential indicators of treatment response heterogeneity in HER2-positive breast cancer.

## Key findings

- One patient showed a favorable response with reduced cell-in-cell structures after neoadjuvant chemotherapy.
- The other patient had limited response with persistent complex cell-in-cell structures.
- Dynamic cell-in-cell patterns may guide individualized treatment planning.

## Abstract

Neoadjuvant chemotherapy (NACT) is commonly applied in human epidermal growth factor receptor 2 (HER2)-positive breast cancer to reduce tumor size and increase the likelihood of breast-conserving surgery. However, predictive markers of response remain limited. We report two cases of HER2-positive breast cancer with divergent responses to NACT, highlighting the dynamics of cell-in-cell structures (CICs). Both patients initially presented with palpable breast masses and biopsy-confirmed HER2-positive invasive carcinoma with frequent CICs. After NACT, one patient achieved a favorable pathological response, accompanied by a reduction in CICs frequency. In contrast, the other patient showed limited response, with persistent complex CICs, which led to an adjustment of the adjuvant regimen. These observations suggest that dynamic cell-in-cell (CIC) patterns may reflect heterogeneous tumor responses to NACT, providing insights for individualized treatment planning.