Metastatic breast tumors from extramammary malignancies
Metastatic breast tumors from extramammary malignancies are quite rare. Characteristics of such tumors are unclear due to a small number of reported cases. During 2012–2019, approximately 3,500 malignant breast tumors were diagnosed with needle biopsy at the hospital and experienced three cases (0.09%) of metastatic extramammary malignancies.

A 59-year-old woman underwent curative surgery for thyroid cancer. After developing lung and ovarian metastases, she visited our department with a mass in her right breast. Ultrasound revealed a 7 mm-sized oval mass. With a high depth–width ratio and abundant blood flow, primary breast cancer was suspected. Core needle biopsy revealed atypical cells with nuclear grooves proliferating in papillary formation. With immunohistochemical examination, her final diagnosis was metastatic thyroid cancer. Imaging suggested malignancies, but it was difficult to distinguish them from primary breast cancer.