Duct: Definition

A duct is a small, tube-like structure that carries fluid from one part of the body to another. Ducts are a normal part of many organs, and they move substances such as bile, urine, sweat, saliva, and breast milk from where they are made to where they are used or released. The word “duct” appears on pathology reports both as normal anatomy and as the starting point of certain cancers, such as ductal carcinoma of the breast. This article explains what a duct is, what it does, and what it can mean when a report describes a duct as dilated, obstructed, or the site where a cancer began.

What is a duct made of, and what does it do?

A duct is lined by epithelial cells, which form a protective inner surface. The shape and size of these cells vary depending on the duct’s location and job. Some ducts also have muscle cells in their walls that help push fluid along. The main job of a duct is to move fluid from a gland or organ, where the fluid is produced, to another part of the body or to the outside. These fluids can include digestive enzymes, bile, saliva, sweat, and other substances the body needs, and ducts help them reach their destination.

Where are ducts found in the body?

Ducts are found throughout the body in several systems:

Digestive system — Bile ducts carry bile from the liver and gallbladder, and pancreatic ducts carry digestive enzymes from the pancreas into the intestine to help with digestion.
Urinary system — The ureters act as ducts that carry urine from the kidneys to the bladder.
Reproductive system — In males, ducts carry sperm from the testicles toward the outside of the body. In females, the fallopian tubes act as ducts that carry eggs from the ovaries toward the uterus.
Skin — Sweat ducts carry sweat from the sweat glands to the surface of the skin.
Salivary glands — Ducts carry saliva from the salivary glands into the mouth.

Not every gland uses a duct. Glands that use ducts are called exocrine glands. Endocrine glands, such as the thyroid and adrenal glands, are “ductless”: they release hormones directly into the bloodstream rather than through a duct.

Which cancers start in ducts?

Cancers that begin in the epithelial cells lining a duct are called carcinomas. Because ducts are found in many organs, the type of cancer depends on the location and function of the duct involved. Some examples include:

Breast cancer — Two of the most common breast cancers start in the ducts that carry milk: ductal carcinoma in situ (DCIS) and invasive ductal carcinoma. In DCIS, the abnormal cells are still contained within the duct and have not spread beyond it. In invasive ductal carcinoma, the cells have grown out of the duct into the surrounding breast tissue and can potentially spread further.
Pancreatic cancer — Pancreatic ductal adenocarcinoma (PDAC) is the most common type of pancreatic cancer and starts in the ducts of the pancreas. It often grows and spreads quickly and is frequently found at a later stage.
Bile duct cancer — Bile duct cancer, also called cholangiocarcinoma, begins in the bile ducts that carry bile from the liver and gallbladder to the small intestine. It is uncommon and is often found at a later stage, when it can be harder to treat.
Salivary gland cancer — Some salivary gland cancers, such as salivary duct carcinoma, begin in the ducts that carry saliva into the mouth.

Each of these cancers begins in the cells lining the ducts of the organ involved. The treatment and outlook depend on the specific type of cancer, its stage at diagnosis, and other features described in the pathology report.

What does it mean if a duct is dilated?

A duct is described as dilated when it is wider than normal. Ducts often become dilated when something, such as a tumor or a stone, blocks one end. Fluid then backs up behind the blockage, causing the duct to fill and stretch. A dilated duct is a clue that prompts the care team to look for the underlying cause.

What does it mean if a duct is obstructed?

An obstructed duct is one that is blocked on the inside, so fluid cannot pass through normally. For example, the duct that leads from the parotid (a large salivary gland in the cheek) to the mouth can become obstructed by a stone, called a sialolith, which may cause pain or swelling in the gland. Depending on where the duct is, an obstruction can be caused by a stone, thickened fluid, inflammation, scarring, or a tumor.

Questions to ask your doctor

Which duct is mentioned in my report, and what does it normally do?
Is the duct described as normal, dilated, or obstructed?
If a duct is dilated or blocked, what is causing it?
Does my report describe a cancer that started in a duct, such as ductal carcinoma?
If so, is the cancer still contained within the duct (in situ) or has it spread beyond it (invasive)?
What further tests or imaging are needed to find the cause?
What are the next steps based on these findings?