Invasive micropapillary carcinoma of mixed breast cancer metastasizing to the cervical region and thyroid: report of a rare case.

While the incidence of metastatic breast cancer tumors in the thyroid gland is extremely rare, invasive micropapillary carcinoma (IMPC) of the breast metastasizing to the thyroid gland is even rarer. There are no known reported cases in which both invasive ductal carcinoma-no special type (IDC-NST) and IMPC existed in the breast tissue and where only the latter metastasized into the thyroid gland. This report details the case of a 59-year-old Chinese woman who was first diagnosed with grade 2 IDC-NST of the breast with involvement of 6 axillary lymph nodes in 2015. However, 5 years later, blood tests revealed increased tumor markers. Imaging revealed multiple solid nodules in both sides of the thyroid gland. Total thyroidectomy and bilateral cervical lymph node dissection were performed; the findings showed both IDC-NST and IMPC in the lymph nodes, but only the latter in the thyroid gland. Immunohistochemically, these tumor cells were positive for estrogen receptor (ER), progesterone receptor (PR), GATA binding protein 3 (GATA3), and mammaglobin, but negative for thyroglobulin (TG) and thyroid transcription factor-1 (TTF-1). Ultimately, the patient was diagnosed as having mixed breast cancer with metastatic IMPC in the thyroid gland, and chemotherapy was continued. Apart from the possibility of IDC-NST in breast tissue metastasizing to the thyroid, metastasis of IMPC of the breast must also be taken into consideration, especially when the patient has a history of breast cancer and thyroid nodules. Accurate diagnosis of metastatic breast carcinoma is vital for precise treatment and can improve the prognosis of patients.