[A Case of Suspected Contralateral Occult Breast Cancer Occurring a Decade Following Breast Cancer Surgery].

A 50-year-old woman was diagnosed with left-sided breast cancer 1 decade previously. The patient underwent left partial mastectomy and sentinel lymph node biopsy, which necessitated axillary dissection. The cancer was classified as cT1cN0M0, indicating Stage Ⅰ, ER-positive, and HER2-negative breast cancer, with no metastatic findings on imaging. The final stages were pT1cN1aM0 and pStage ⅡA. Postoperatively, the patient received TC(docetaxel+cyclophosphamide) chemotherapy, total breast irradiation, and tamoxifen for 10 years. Positron emission tomography/computed tomography conducted 10 years postoperatively revealed an enlarged right axillary lymph node with an SUVmax of 1.3. A Class Ⅴ diagnosis was made via US-guided fine-needle aspiration cytology. No additional metastases were suspected and magnetic resonance imaging revealed no bilateral mass lesions in the breasts. Consequently, a histological diagnosis was required to ascertain whether the lesion was recurrent, which led to right axillary lymph node dissection. The specimen was identified as a metastatic carcinoma(GATA-3-positive, ER-positive, and HER2-positive). Considering the different subtypes and absence of other lesions, the patient was considered to have latent breast cancer as a new lesion. Chemotherapy, including anti- HER2 therapy, radiotherapy, and endocrine therapy, was administered as adjuvant treatment.