Background: A complex breast lesion comprising both fibroadenoma and carcinoma is very challenging to diagnose early in the disease.Case Presentation: The patient was a 76-year-old woman who was followed at our clinic for 8 years because of a 7-mm tumor, which was determined to be fibroadenoma on a core needle biopsy specimen. However, another core needle biopsy was performed because part of the margin of the tumor was obscure. A histopathological examination revealed that the second specimen contained malignant cells. The patient underwent breast-conserving surgery and a sentinel lymph node biopsy. The histopathological examination found both breast carcinoma and fibroadenoma, with some of the 2 lesions overlapping. The sentinel lymph node specimen showed micrometastases. The patient’s postoperative course was uneventful. Conclusions: The findings in our patient suggest that careful examination of the margin of the breast fibroadenoma was essential for early detection of the breast carcinoma.
