# Stabilization of Symptomatic Bone Marrow Metastases in Breast Cancer: A Case Report of Two Patients Treated With Trastuzumab Deruxtecan

## TL;DR

Two patients with rare bone marrow metastases from breast cancer showed full recovery after treatment with trastuzumab deruxtecan.

## Contribution

Demonstrates the efficacy of T-DXd in treating HER2-low breast cancer with bone marrow infiltration.

## Key findings

- Trastuzumab deruxtecan led to rapid and complete hematologic recovery in two patients.
- Patients remained stable and free of marrow disease for over two years.
- T-DXd shows potential as a therapeutic option for HER2-low breast cancer with bone marrow metastases.

## Abstract

Symptomatic bone marrow infiltration (BMI) in breast cancer is exceedingly rare and often associated with poor prognosis due to cytopenias and limited therapeutic options. We report two elderly patients with a history of hormone receptor-positive, HER2-negative breast cancer who presented with pancytopenia years after initial treatment. Bone marrow biopsy revealed HER2-low metastatic relapse. Treatment with trastuzumab deruxtecan (T-DXd) led to unexpectedly rapid and complete hematologic recovery after only two cycles, with both patients remaining stable and free of marrow disease for over two years. These cases demonstrate the remarkable efficacy and tolerability of T-DXd in this uncommon and challenging clinical scenario, highlighting its potential as a valuable therapeutic option in HER2-low breast cancer with BMI.