A 54-year-old postmenopausal woman presents with a screening-detected spiculated mass and microcalcifications in the left upper outer quadrant. Core needle biopsy shows infiltrating cords, nests, and tubules of pleomorphic malignant epithelial cells with vesicular nuclei, prominent nucleoli, mitotic activity, and a dense desmoplastic stromal reaction. Immunohistochemistry reveals ER 95% strong nuclear staining, PR 80% positive, HER2 3+ membranous overexpression (ISH amplification ratio 6.2), and Ki-67 35%. The structure marked **A** in the diagram represents the histologic diagnosis. Which of the following best characterizes the defining microscopic feature that distinguishes this lesion from invasive lobular carcinoma?

Explanation

## Why "Presence of infiltrating glands, cords, and sheets of malignant epithelial cells with desmoplastic stromal reaction" is right Invasive ductal carcinoma, no special type (IDC-NST), marked as **A**, is defined by malignant epithelial cells arranged in glands, cords, sheets, or single cells infiltrating into the stroma with a prominent desmoplastic (myofibroblastic) stromal response. This desmoplasia is responsible for the firm, gritty texture and stellate mammographic appearance. IDC-NST is the most common form of invasive breast cancer, accounting for 75–80% of cases. The combination of variable architectural patterns (glands, cords, sheets) and the characteristic desmoplastic stromal reaction is the hallmark that distinguishes it from special-type carcinomas. (Robbins 10e, Breast; ASCO/CAP HER2 Testing 2023) ## Why each distractor is wrong - **Single-file infiltration of tumor cells with loss of E-cadherin expression**: This describes invasive lobular carcinoma (ILC), marked as **B**, not IDC-NST. ILC has a distinctive single-file pattern and E-cadherin loss, which is a different histologic subtype with different prognosis and clinical behavior. - **Well-formed tubular structures with excellent prognosis and low nuclear grade**: This describes tubular carcinoma, a special-type subtype of invasive breast cancer with an excellent prognosis. Tubular carcinoma is not the same as IDC-NST; it is a distinct special-type variant with better outcomes. - **Extracellular mucin lakes surrounding nests of tumor cells with predominantly elderly presentation**: This describes mucinous (colloid) carcinoma, another special-type variant that occurs predominantly in elderly patients. It is not the defining feature of IDC-NST. **High-Yield:** IDC-NST is the "default" invasive breast cancer diagnosis—it is defined by variable architectural patterns (glands, cords, sheets) and desmoplasia, and accounts for ~75–80% of invasive breast cancers. All other subtypes are "special types" with distinct histologic and prognostic features. [cite: Robbins 10e — Breast; ASCO/CAP HER2 Testing 2023]