# Trastuzumab Deruxtecan Significantly Reduces Human Epidermal Growth Factor Receptor 2-Positive Pancreatic Metastasis from Advanced Breast Cancer: A Case Report

## TL;DR

A patient with HER2-positive breast cancer and pancreatic metastasis achieved complete response with trastuzumab deruxtecan, a new treatment option.

## Contribution

Demonstrates the effectiveness of trastuzumab deruxtecan in treating rare HER2-positive pancreatic metastases from breast cancer.

## Key findings

- Trastuzumab deruxtecan achieved a complete response in a patient with HER2-positive pancreatic metastasis.
- The treatment provided 14 months of progression-free survival without severe adverse events.
- Endoscopic ultrasonography-guided fine-needle biopsy is recommended for diagnosing pancreatic metastases.

## Abstract

Pancreatic metastases from breast cancer are rare, with only 61 reported cases. They are often difficult to differentiate from primary pancreatic cancer based on clinical presentation and imaging. The median overall survival after the diagnosis of pancreatic metastases has been reported to be short (17 months).

A 50-year-old woman was diagnosed with human epidermal growth factor receptor 2 (HER2)-positive advanced right breast cancer, with right axillary lymph node and multiple bone metastases. After palliative irradiation for thoracolumbar metastases, the patient received pertuzumab with trastuzumab plus docetaxel as 1st-line treatment, with a partial response for 9 months. Second-line trastuzumab emtansine maintained stable disease for 14 months (25 months from the initial diagnosis). A follow-up CT scan revealed a pancreatic tumor measuring 48 × 28 mm, which was diagnosed as a metastasis from breast cancer (estrogen receptor: 0%; progesterone receptor: 0%; HER2: 3+; Ki67: 60%) by endoscopic ultrasonography-guided fine-needle biopsy (FNB). MRI revealed a mass in the pancreatic head. Three brain metastases were also detected using head MRI. After stereotactic irradiation of the cerebellar metastases, trastuzumab deruxtecan (T-DXd) was started as 3rd-line treatment, and the pancreatic metastasis was markedly reduced, and a complete response (CR) was achieved, which was effective for 14 months. No severe adverse events were observed during the T-DXd treatment. Capecitabine and lapatinib were administered as 4th-line treatment owing to new pleural dissemination. The patient died of worsening cancerous lymphangitis of the lungs 23 months after the diagnosis of pancreatic metastasis.

Our patient with HER2-positive pancreatic metastasis achieved CR with marked shrinkage using T-DXd and a progression-free survival of 14 months. FNB should be performed to diagnose possible pancreatic metastases if a pancreatic tumor is detected during breast cancer treatment.