Case series Patients: Female, 48-year-old • Female, 47-year-old Final Diagnosis: Metastatic breast cancer Symptoms: Edema of left upper limb • no symptom Medication: — Clinical Procedure: Median sternotomy • small incisional biopsy Specialty: Surgery Objective: Challenging differential diagnosis Background: Breast cancer is becoming a common disease in women. It progresses slowly and may recur after a long time. Therefore, when a tumor is found in the chest of a patient with a history of breast cancer, an immediate concern is whether it is a primary tumor or a metastatic tumor. However, mediastinal metastasis is extremely unlikely to occur before lung metastasis, and breast cancer is not likely to have a solitary mediastinal metastasis. Additionally, patients should not undergo invasive procedures unnecessarily, so careful consideration is required. Case Reports: We present 2 cases. In case 1, a 48-year-old woman with a history of breast cancer had a mediastinal tumor. Based on imaging findings, cystic thymoma was suspected. Thoracoscopic intraoperative rapid biopsy showed a lymphocyte-predominant tumor tissue; therefore, the tumor was resected via a median sternotomy. The final pathological diagnosis was breast cancer metastasis. In case 2, a 47-year-old woman who underwent breast cancer resection 15 years earlier was referred for upper limb edema. Based on imaging findings, a left medial vein occlusion due to mediastinal tumor was diagnosed. Our experience in case 1 suggested that a biopsy alone should be performed first. A tumor biopsy was performed through a small transverse neck incision in case 2, and the final diagnosis was metastatic breast cancer of the mediastinum. Conclusions: In patients with a suspected primary mediastinal tumor on imaging, the possibility of a metastatic tumor should be considered if they have a history of breast cancer, regardless of how long in the past it was.