12.3 DCIS, IDC, and Invasive Lobular Carcinoma

Malignant Disease the Technologist Must Recognize

ARRT Mammography Procedures 1.E.4 expects you to know how common malignant lesions look on a mammogram and which named histology those looks suggest. Histology still comes from a biopsy. Your job is to produce images that make DCIS calcifications countable on magnification, that show a spiculated mass without motion blur, and that do not crop the thin distortion of lobular carcinoma off the posterolateral MLO.

Keep the in situ versus invasive line clean. In situ disease has not broken through the basement membrane into supporting tissue. Invasive (infiltrating) disease has. That biologic line — not how "scary" the calcifications look — is what separates DCIS from IDC.

Ductal Carcinoma in Situ (DCIS)

DCIS (intraductal carcinoma) is a non-invasive neoplastic proliferation inside the ducts. The American Cancer Society describes it as non-invasive or pre-invasive: the cells have not grown into surrounding breast tissue. It is still treated as a malignant process because it can be extensive, can harbor microinvasion, and can recur as invasive disease if untreated.

Mammographic appearance

The dominant mammographic presentation of DCIS is calcifications, not a lump. Using ACR BI-RADS morphology language:

• Fine pleomorphic calcifications — irregular, varying in size and shape, smaller and more irregular than typically benign popcorn or dystrophic calcs
• Fine linear and fine linear branching calcifications — casting along a duct, as if the duct were filled with a thin, broken, branching cast

Suspicious distribution matters as much as shape. Linear and segmental distributions follow a duct or a lobe and raise concern for DCIS. Grouped (clustered) pleomorphic calcs are a common biopsy target. Diffuse scattered round calcs are more often benign.

DCIS can also appear as a mass, asymmetry, architectural distortion, or Paget-type nipple change (Section 12.4) without calcs, but "DCIS = suspicious calcifications" is the association the exam leans on. Magnification views (and tomosynthesis or synthesized 2D, per protocol) characterize morphology; specimen radiography after stereo biopsy must show that the target calcifications are in the cores.

Typically benign calcifications you should not confuse with DCIS include vascular (tram-track), skin, large rod-like secretory, rim oil-cyst, dystrophic, and popcorn (involuting fibroadenoma). The technologist who can name those typically benign forms is less likely to treat every white speck as an emergency — and less likely to miss true pleomorphic/linear casts.

High-grade DCIS is more often the fine linear branching / casting type; low-grade DCIS more often shows amorphous or less dramatic calcs. Grade is still pathology. Do not invent a percentage of DCIS cases that "present as calcifications" unless an ACS/ACR/NCI source is in front of you; saying often or classically is the honest exam wording.

Invasive / Infiltrating Ductal Carcinoma (IDC)

Invasive ductal carcinoma is also called infiltrating ductal carcinoma. The American Cancer Society states it is the most common type of breast cancer and that about 8 in 10 invasive breast cancers are IDC. ACS also notes that invasive ductal carcinoma makes up about 70–80% of all breast cancers on its types-of-breast-cancer overview — use that ACS range rather than a homemade national share.

IDC starts in ductal epithelium and invades stroma. Because it often incites a desmoplastic (fibrotic) reaction, it is the histology that produces the textbook spiculated mass.

Mammographic appearance

IDC is a shape-shifter. Expect any of:

• A mass — irregular shape, spiculated or indistinct margins, often high density relative to equal-density fibroglandular tissue
• Architectural distortion without a discrete mass
• Suspicious calcifications (DCIS may be the in situ component of an invasive tumor; calcs and a mass together are a classic pairing)
• A developing asymmetry that is new compared with priors

Palpable IDC may still be mammographically subtle in dense tissue. That is why a palpable-lump marker, diagnostic views, and ultrasound of the clinical finding are not optional because "the screening views looked okay." Axillary adenopathy on the MLO (loss of fatty hila, round dense nodes) can accompany invasive disease; document it, do not crop it.

IDC is common enough that when an exam question says "most common invasive breast cancer" or "infiltrating ductal," ACS's 8 in 10 invasive figure is the citable fact. Do not quote a practice's local biopsy mix as if it were SEER.

Invasive Lobular Carcinoma (ILC)

Invasive lobular carcinoma is the second most common histologic type of invasive breast cancer. It grows as single-file cells through stroma, often with little desmoplasia, so it may not form a discrete lump. Patients and clinicians both miss it on palpation more easily than a hard IDC mass. The American Cancer Society notes that, compared with other invasive carcinomas, ILC is more likely to affect both breasts, and that about 1 in 5 women with ILC have cancer in both breasts at the time they are diagnosed.

SEER's dedicated ILC stat facts page describes ILC as about 15% of new breast cancer diagnoses and the second most common histologic type, with invasive ductal carcinoma representing over 80% of U.S. diagnoses. If an item asks for a cited national share, that ACS/SEER language is the safe source; do not invent a more precise "exactly X%" from memory.

Mammographic appearance — why ILC is the easy-to-miss cancer

• Architectural distortion or focal asymmetry rather than a round mass
• Density that is only equal to parenchyma, not obviously high-density
• A shrinking breast or decreasing size on sequential exams (less common, but taught)
• Mammographically occult disease: the 2D mammogram can be negative while a palpable thickening or MRI abnormality is real

Digital breast tomosynthesis (DBT) reduces overlapping tissue and is conceptually better than 2D alone at showing distortion. Contrast-enhanced breast MRI is conceptually the most sensitive tool for extent of ILC and for looking at the contralateral breast. The mammography technologist does not interpret MRI, but exam items expect you to know why the radiologist may recommend it after a subtle distortion or a known ILC diagnosis.

Positioning quality is a safety issue here. A weakly compressed MLO that drops the inframammary fold and the posterolateral breast can hide the only clue. Compare with priors hung the same way. If the patient has a thickening the surgeon can feel and you cannot find a mass, that discordance is a diagnostic workup, not a "negative screen."

Classic ILC is often E-cadherin negative on the pathology stain (the same family as LCIS). That is a microscope fact; the imaging correlate is the infiltrative, non-mass pattern, not a colored stain on the mammogram.

Putting the Three Together at the Console

Exam trap: "Most common" on this blueprint means IDC among invasive cancers (ACS: about 8 in 10 invasive). DCIS is common as a screening-detected calcification disease, but it is not invasive. ILC is not rare in the sense that you can forget it; it is the histology that makes a negative mammogram with a positive physical exam believable. If the question stem is "single-file infiltration, may be occult, better seen with DBT/MRI," the answer is ILC, not DCIS.