# Breast Cancer Metastasis to the Upper Arm: A Rare Case and Review of the Literature

## TL;DR

This paper reports a rare case of breast cancer metastasis to the upper arm and discusses the challenges in diagnosis and treatment.

## Contribution

The paper presents a new case of upper arm metastasis and reviews the limited existing literature on this rare condition.

## Key findings

- Breast cancer metastasis to the upper arm is extremely rare, with only 10 reported cases.
- Metastasis near nerves complicates diagnosis and surgical resection.
- Early diagnosis and multimodal treatment are crucial for poor-prognosis soft-tissue metastases.

## Abstract

Soft tissue metastasis of breast cancer is very rare. Specifically, metastasis to the upper arm has only been reported in 10 cases, including this one, to the best of our knowledge. Distant lymph node metastasis of breast cancer is also infrequent, with only one documented case of brachial lymph node metastasis.

We report a case of soft tissue or lymph node metastasis in the right upper arm, diagnosed 3 years after surgery for right breast cancer. The patient, a 79-year-old woman, was receiving postoperative hormone therapy for right breast cancer. She presented with edema and numbness in the right upper arm during a routine follow-up examination 3 years post-surgery. Ultrasonography revealed a 20 mm mass in the right brachial muscle, which was diagnosed as adenocarcinoma via fine-needle aspiration, suggesting breast cancer metastasis. Further examination showed that the mass was in close proximity to the nerves and veins, but no definitive evidence of invasion was observed. Surgical resection was performed. The tumor was particularly adherent to the nerve, complicating resection. While no gross residual tumor was noted, histopathological analysis indicated positive surgical margins. After surgery for soft tissue or lymph node metastasis, the patient continued hormone therapy postoperatively. However, approximately 1 year and 3 months later, tumor recurrence at the resection site and skin metastasis were observed. Consequently, hormone therapy was modified, and oral cyclin-dependent kinase 4/6 (CDK4/6) inhibitors were initiated.

Metastasis of breast cancer to soft tissue and the brachial lymph nodes is extremely rare. Diagnosis may be delayed in asymptomatic cases, and when metastasis is in close proximity to surrounding structures, such as the nerves, histological diagnosis can be challenging. Metastatic soft-tissue tumors are associated with a poor prognosis; therefore, early diagnosis and appropriate multimodal treatment are crucial.