Carcinoma: Definition

Carcinoma is a type of cancer that starts in epithelial cells — the cells that line the surfaces and cavities of the body, both inside and out. Epithelial cells cover the skin, line the digestive tract, and form the inner surfaces of organs such as the lungs, breasts, prostate, and cervix. Because epithelial cells are found almost everywhere in the body, carcinoma is the most common category of cancer overall. The word carcinoma in a pathology report tells you the type of cell the cancer came from — not where it is located or how serious it is.

What are the main types of carcinoma?

Carcinomas are named based on the specific type of epithelial cell they arise from. The most common types include:

Adenocarcinoma — arises from glandular cells that line organs and produce substances such as mucus or digestive juices. It is the most common type of carcinoma in many organs, including the colon, lung, breast, prostate, stomach, pancreas, and uterus (endometrium).
Squamous cell carcinoma — arises from flat squamous cells that form the surface layers of the skin and line organs such as the mouth, throat, esophagus, lungs, and cervix. It is the most common type of skin cancer and a common cancer of the head and neck.
Adenosquamous carcinoma — contains both glandular and squamous cell components within the same tumor. Seen in several organs including the lung, cervix, and pancreas.
Neuroendocrine carcinoma — arises from specialized neuroendocrine cells that have characteristics of both hormone-producing and epithelial cells. Most commonly found in the lungs and gastrointestinal tract.
Urothelial carcinoma — arises from the transitional cells lining the urinary bladder, ureters, and renal pelvis.
Basal cell carcinoma — arises from cells at the base of the skin’s outer layer. It is the most common cancer overall, though it rarely spreads beyond the skin.

What does carcinoma look like under the microscope?

When a pathologist examines a carcinoma, the cells typically show features of malignancy — enlarged and irregular nuclei, abnormal growth patterns, and varying degrees of resemblance to the normal tissue they came from. The degree of resemblance to normal tissue is described as differentiation and forms the basis of tumor grade:

Well-differentiated carcinoma — the cancer cells still closely resemble the normal epithelial cells they came from. These tumors tend to behave less aggressively.
Poorly differentiated carcinoma — the cancer cells look very abnormal and bear little resemblance to normal tissue. These tumors tend to be more aggressive.
Undifferentiated carcinoma — the cells look so abnormal that their origin can no longer be determined by appearance alone, and additional testing is required.

Immunohistochemistry (IHC) is frequently used to confirm the diagnosis of carcinoma and identify the specific type, using markers characteristic of different epithelial cell types.

What is the difference between carcinoma in situ and invasive carcinoma?

One of the most important distinctions in a carcinoma diagnosis is whether the cancer is in situ or invasive:

Carcinoma in situ — the cancer cells are confined to the epithelial layer where they started. They have not broken through the basement membrane — the thin boundary that separates the epithelium from the deeper tissue. Because they cannot access blood vessels or lymphatics, in situ carcinomas cannot spread to other parts of the body. They are considered pre-invasive and are usually curable with surgery.
Invasive carcinoma — the cancer cells have broken through the basement membrane and grown into the surrounding tissue. From there, they can potentially access blood vessels and lymph nodes, raising the risk of spread to other parts of the body. Most carcinomas in pathology reports are invasive.

Are all cancers carcinomas?

No — carcinoma is one of several major categories of cancer, distinguished by the cell type of origin. Other important cancer types include:

Sarcoma — cancer arising from connective tissues such as bone, muscle, fat, or cartilage.
Lymphoma — cancer arising from lymphocytes, the immune cells of the lymphatic system.
Melanoma — cancer arising from melanocytes, the pigment-producing cells of the skin.
Leukemia — cancer arising from blood-forming cells in the bone marrow.

Identifying which category a cancer belongs to is essential because each type behaves differently and requires different treatment. This is one reason pathologists use IHC and other tests alongside routine microscopic examination.

What does a carcinoma diagnosis mean?

A diagnosis of carcinoma confirms that a cancer of epithelial origin has been identified. The word “carcinoma” alone does not tell you how serious the cancer is — that depends on the specific type, location, grade, and stage, as well as the results of any additional biomarker or molecular testing. Some carcinomas, such as early-stage basal cell carcinoma or papillary thyroid carcinoma, have an excellent prognosis. Others, such as pancreatic adenocarcinoma or small cell carcinoma of the lung, are significantly more challenging to treat. Your oncologist will explain what your specific carcinoma diagnosis means for your treatment and outlook.

Questions to ask your doctor

What type of carcinoma has been diagnosed, and which organ did it start in?
Is it in situ or invasive — and what does that mean for my treatment?
What is the grade of my carcinoma, and how does that affect the treatment approach?