Correct: B

Histologic Description: This is an infiltrative ductal process associated with an irregular border and mild stromal desmoplasia. The cells form tubules and cribriform structures and are associated with intracellular and extracellular eosinophilic secretions. The neoplasm demonstrates minimal immunoreactivity for estrogen receptor and progesterone receptor, unlike most low nuclear grade breast cancers. FISH demonstrates rearrangement of the ETV6 gene, supporting the diagnosis of invasive secretory carcinoma.
Differential Diagnosis: Conventional invasive ductal carcinoma would be distinctly unusual in this age group and gender, and does not harbor ETV6 gene rearrangements. Florid papillomatosis of the nipple would have an overall circumscribed appearance, and have bilayered epithelium containing both myoepithelial and ductal cells. Gynecomastia would feature prominent periductal edema and fibrosis, associated with micropapillary duct hyperplasia.
Invasive secretory carcinoma is a special favorable type of breast cancer associated with the ETV6-NTRK3 gene fusion. The latter is also seen in a variety of other neoplasms, including infantile fibrosarcoma, cellular congenital mesoblastic nephroma, and mammary analog secretory carcinoma of salivary glands. The latter is histologically identical to secretory carcinoma of the breast.