A 51-year-old woman has a left breast mass; mammogram and ultrasound are normal; biopsy shows cancer. What is the most likely histologic type?

• A. Infiltrating ductal
• B. Fibroadenoma
• C. Ductal carcinoma in situ
• D. Infiltrating lobular

Answer: (D)

Some breast cancers grow in a way that can escape routine imaging. Infiltrating lobular carcinoma tends to invade the breast tissue in single-file strands of cells and does not usually form a discrete mass or strong desmoplastic reaction. Because of this diffuse, subtle growth pattern, mammography and ultrasound can be normal even when cancer is present, especially in middle-aged women. Biopsy revealing cancer in this context points to infiltrating lobular carcinoma. Histology would show small, uniform cells infiltrating the stroma in an Indian-file pattern with loss of cell–cell adhesion (often due to loss of E-cadherin), rather than glands forming a mass with desmoplasia as seen in infiltrating ductal carcinoma. Ductal carcinoma in situ is noninvasive and commonly presents with calcifications on imaging, and a fibroadenoma is benign and typically occurs in younger women.