Invasive ductal carcinoma (IDC) is the most common type of breast cancer, accounting for about 80% of all breast cancer diagnoses. It starts in the cells lining the milk ducts and, unlike earlier-stage conditions, has spread beyond the duct wall into surrounding breast tissue. The remaining 20% of cases are split among several less common types, with invasive lobular carcinoma being the next most frequent at 10% to 15%.
How Invasive Ductal Carcinoma Develops
Your breast contains a network of milk ducts that carry milk to the nipple. IDC begins in the cells lining these ducts. In many cases, it starts as a precancerous condition called ductal carcinoma in situ (DCIS), sometimes referred to as stage 0 breast cancer. At the DCIS stage, abnormal cells are contained entirely within the duct and haven’t broken through its walls.
The duct is surrounded by a protective layer of specialized cells and a thin membrane that act as a barrier. When that barrier breaks down, cancer cells push through into the surrounding breast tissue. Once they cross that membrane, they gain the ability to spread to lymph nodes and eventually to distant organs. This transition from contained to invasive is what distinguishes IDC from DCIS. Not every case of DCIS progresses to invasive cancer, but long-term studies show that some untreated cases do.
Who Gets IDC
IDC typically affects women age 55 or older, though it can occur at younger ages. It is also the most common form of breast cancer in men, who account for a small fraction of overall cases. No single demographic group is immune, but risk increases with age, family history of breast cancer, and certain genetic mutations.
Molecular Subtypes and Why They Matter
Not all IDC behaves the same way. Doctors classify breast cancers by whether the tumor cells have receptors for hormones (estrogen and progesterone) and whether they overproduce a growth-promoting protein called HER2. These molecular subtypes determine which treatments are most effective and give a clearer picture of prognosis than the diagnosis of IDC alone.
The breakdown across all female breast cancers, based on recent national cancer registry data from 2019 to 2023, looks like this:
- Hormone receptor-positive, HER2-negative (HR+/HER2-): about 70% of cases. These tumors grow in response to estrogen or progesterone, making them responsive to hormone-blocking therapies. This is by far the most common subtype.
- Hormone receptor-negative, HER2-negative (triple-negative): about 11% of cases. These tumors lack all three common targets, which limits treatment options and generally carries a more aggressive course.
- Hormone receptor-positive, HER2-positive (HR+/HER2+): about 9% of cases. These respond to both hormone therapy and HER2-targeted drugs.
- Hormone receptor-negative, HER2-positive (HR-/HER2+): about 4% of cases. These are treated primarily with HER2-targeted therapies.
Because roughly 70% of all breast cancers are hormone receptor-positive and HER2-negative, the majority of people diagnosed with IDC will be candidates for hormone-blocking treatments, which are taken as pills for several years after initial treatment and significantly reduce recurrence risk.
How IDC Is Found
IDC is often the type of breast cancer detected on routine mammograms, where it may appear as an irregular mass or cluster of tiny calcium deposits (calcifications). On ultrasound, these tumors typically show up as irregularly shaped, darker-than-normal masses that cast a shadow behind them. Many people first notice IDC as a hard, painless lump during a self-exam or clinical breast exam, though not all tumors are large enough to feel.
After imaging raises suspicion, a biopsy confirms the diagnosis and determines the molecular subtype. The biopsy also reveals the tumor grade, which describes how abnormal the cells look under a microscope and how quickly they’re dividing.
Survival Rates by Stage
Prognosis for breast cancer depends heavily on how far it has spread at the time of diagnosis. The most recent five-year relative survival rates from the National Cancer Institute, covering 2013 to 2019, apply across breast cancer types:
- Localized (cancer is still within the breast): 99.3%
- Regional (spread to nearby lymph nodes): 86.3%
- Distant (spread to other parts of the body): 31%
The vast majority of breast cancers are caught at the localized or regional stage, which is a major reason overall breast cancer survival rates are high. Routine screening mammograms, generally recommended starting between ages 40 and 50 depending on risk factors, are the primary tool for catching IDC early.
Invasive Lobular Carcinoma: The Second Most Common Type
Invasive lobular carcinoma (ILC) accounts for 10% to 15% of breast cancer cases. It starts in the milk-producing glands (lobules) rather than the ducts. What makes ILC distinctive is the way it grows. The tumor cells lack a protein called E-cadherin that normally helps cells stick together. Without it, lobular cancer cells spread through breast tissue in thin, single-file lines rather than clumping into a firm mass. This growth pattern means ILC tumors often don’t form a noticeable lump and are harder to detect on both mammograms and self-exams.
Because of this, ILC is frequently diagnosed when tumors are already larger than 2 centimeters. About 90% of lobular breast cancers are hormone receptor-positive and HER2-negative, making hormone therapy the cornerstone of treatment. ILC also has some unusual characteristics when it spreads. While both IDC and ILC can metastasize to bones, brain, liver, and lungs, lobular cancer has a tendency to spread to less typical sites: the stomach, intestines, ovaries, uterus, and the lining of the abdomen. Lobular tumors also have a pattern of recurring many years after the original diagnosis, sometimes a decade or more later, which makes long-term follow-up especially important.
DCIS: The Non-Invasive Precursor
Ductal carcinoma in situ is worth understanding because it shares a name and origin with IDC but sits at a fundamentally different stage. DCIS is classified as stage 0 breast cancer. The abnormal cells are still entirely inside the milk duct and have not broken through to surrounding tissue, so DCIS cannot spread to lymph nodes or other organs on its own. It is often found on mammograms as a cluster of calcifications before any symptoms develop.
DCIS is typically treated to prevent it from progressing to invasive cancer. Treatment usually involves surgery to remove the affected area, sometimes followed by radiation. The prognosis is excellent, with survival rates near 100%. Not every case of DCIS will become IDC if left alone, but there is currently no reliable way to predict which cases will and which won’t, so treatment is standard practice.