Carcinoma is cancer that arises from epithelial cells, the cells that line the surfaces of your organs, skin, and internal passageways. It is the most common category of cancer in humans, accounting for the vast majority of diagnoses that fall under the broader umbrella of “cancer.” If you or someone you know has been diagnosed with a cancer whose name ends in “-carcinoma,” understanding what that suffix means is the first step to making sense of the diagnosis, the staging, and the treatment options that follow.
Why the Word Matters
Cancer is not one disease. It is hundreds of diseases grouped by a shared trait: cells growing out of control. The medical system classifies cancers primarily by the type of cell they start in, and that classification determines almost everything about how the disease behaves, how it is staged, and how it is treated. Carcinoma specifically refers to cancers originating in epithelial cells.1PubMed. The pathologic diagnosis of carcinoma in various tissues Epithelial cells form the linings and coverings of the body: your skin, the inside of your lungs, the walls of your colon, the ducts in your breast, the lining of your bladder. Because epithelial tissue is everywhere and is constantly exposed to environmental damage, carcinomas are far more common than other major cancer categories.
Those other categories include sarcomas (cancers of bone, muscle, fat, and connective tissue), lymphomas (cancers of the lymphatic system), leukemias (cancers of blood-forming cells), and cancers of the central nervous system. When a doctor says “carcinoma,” they are telling you where the cancer started at the cellular level, not where it was found in the body. A tumor discovered in the liver, for instance, might be a carcinoma that spread there from the colon, or it could be a hepatocellular carcinoma that originated in the liver’s own epithelial cells. The distinction shapes every clinical decision.
How Carcinomas Begin
Epithelial cells divide regularly to replace cells that wear out, especially in high-turnover tissues like the gut lining and the skin. That constant division creates opportunities for genetic errors. When the right combination of errors accumulates in a single cell, it can begin growing without the usual brakes. One model of how this works describes carcinogenesis as the result of epithelial cells undergoing a shift away from their normal, specialized state toward a less organized, more mobile cell type with cancerous potential.2PubMed Central. Revisiting Epithelial Carcinogenesis In plain terms, epithelial cells lose their identity and start behaving more like the loosely organized cells found in connective tissue, gaining the ability to move and invade neighboring structures.
This process does not happen overnight. Most carcinomas develop over years or even decades, passing through recognizable pre-cancerous stages. A cluster of abnormal-looking cells that has not yet crossed a critical boundary called the basement membrane is typically called carcinoma in situ, sometimes shortened to CIS. The basement membrane is a thin sheet of proteins that separates epithelial tissue from the deeper connective tissue underneath it. As long as abnormal cells stay on the epithelial side of that barrier, the growth is classified as non-invasive and is generally considered curable.3PubMed Central. Force-dependent breaching of the basement membrane
In Situ Versus Invasive Carcinoma
The distinction between in situ and invasive carcinoma is one of the most clinically important dividing lines in cancer medicine. In situ carcinoma has not broken through the basement membrane. It has no access to blood vessels or lymphatic channels, which means it has no route to spread elsewhere. At this stage, removing or destroying the abnormal cells is often curative. Screening programs for cervical, breast, and skin cancers are designed in large part to catch carcinomas at this pre-invasive stage.
Once cells push through the basement membrane and infiltrate the underlying tissue, the carcinoma is classified as invasive.4Journal of Clinical Pathology. Defining invasion in breast cancer: the role of basement membrane Invasive does not necessarily mean the cancer has already spread to distant sites, but it means that spread is now biologically possible. Invasive carcinoma cells can enter lymphatic vessels and blood vessels, travel to lymph nodes or distant organs, and establish new tumors there. Lymph nodes are the most common first stop for spreading carcinoma cells.5PubMed Central. Progression of Metastasis through Lymphatic System That is why doctors check nearby lymph nodes during staging: their status helps predict how far the disease may have traveled.
The cellular mechanism behind invasion involves a process in which carcinoma cells acquire characteristics that let them move and migrate, shifting from a tightly packed epithelial arrangement to a more mobile form.6Precision Medication. Epithelial-mesenchymal transition in cancer metastasis: Molecular mechanisms, microenvironmental regulation, and therapeutic targeting This shift helps explain why some carcinomas are aggressive while others stay relatively contained for long periods.
Common Types of Carcinoma
Because epithelial cells come in several varieties, carcinomas are further subdivided based on the specific epithelial cell type they resemble under a microscope. The names you will encounter most often include:
- Adenocarcinoma: Arises from glandular epithelial cells, the type that produces mucus, digestive juices, or other fluids. This is the most common subtype and includes most cancers of the breast, prostate, colon, lung (the non-squamous kind), pancreas, and stomach.
- Squamous cell carcinoma: Arises from flat, scale-like cells found on the skin and lining the mouth, throat, esophagus, lungs, and cervix. On the skin, squamous cell carcinomas range widely in aggressiveness depending on their microscopic appearance and thickness.7PubMed. Histologic subtyping and malignancy assessment of cutaneous squamous cell carcinoma
- Basal cell carcinoma: The most common cancer in humans, arising from cells in the deepest layer of the skin’s outer covering. It grows slowly and very rarely spreads to distant sites, making it the least dangerous of the major carcinomas.
- Transitional cell carcinoma: Arises from the specialized epithelium that lines the bladder, ureters, and parts of the kidney. These cells are sometimes called urothelial cells, and this cancer is increasingly referred to as urothelial carcinoma.8PubMed Central. Uroplakins, specific membrane proteins of urothelial umbrella cells, as histological markers of metastatic transitional cell carcinomas
Less common subtypes include renal cell carcinoma (kidney), hepatocellular carcinoma (liver), and various rare skin carcinomas. Some carcinomas show mixed features, combining glandular and squamous patterns, which pathologists describe as adenosquamous carcinoma. The subtype matters because different epithelial cell types respond differently to chemotherapy, radiation, and newer targeted treatments.
How Carcinomas Are Identified
Diagnosing a carcinoma typically starts with a biopsy: a small sample of tissue removed and examined under a microscope by a pathologist. The pathologist looks at cell shape, arrangement, and how much the cells resemble normal tissue. A well-differentiated carcinoma still looks somewhat like the tissue it came from; a poorly differentiated one has lost most of its normal features and can be harder to classify.
When the microscopic appearance alone is not enough to determine the origin of a tumor, pathologists turn to a technique called immunohistochemistry, which uses antibodies to detect specific proteins on or inside tumor cells. One of the most broadly useful markers is cytokeratin, a protein found in epithelial cells of all types.9PubMed Central. Useful Immunohistochemical Markers of Tumor Differentiation If a tumor stains positive for cytokeratin, it is almost certainly a carcinoma rather than a sarcoma or lymphoma. From there, different subtypes of cytokeratin can help narrow down which organ the carcinoma came from.10PubMed Central. Immunohistochemical distinction of human carcinomas by cytokeratin typing with monoclonal antibodies
This detective work becomes especially critical when a carcinoma shows up as a metastasis, a secondary tumor in a location far from where the cancer began. A tumor found in the liver might have started in the colon, the lung, or the breast. Panels of immunohistochemical markers now allow pathologists to accurately classify the origin of most of these tumors, even when the cancer cells look quite different from the normal tissue they came from.11PubMed Central. Immunohistochemistry for Diagnosis of Metastatic Carcinomas of Unknown Primary Site
Carcinoma of Unknown Primary
Sometimes, despite thorough testing, doctors cannot identify where a carcinoma started. This situation is called carcinoma of unknown primary, or CUP. It means metastatic cancer has been found, but no primary tumor is identifiable despite imaging and pathology workups.12PubMed Central. Carcinoma of unknown primary origin CUP is not a single disease but a diverse group of cancers that share the frustrating feature of a hidden origin. It accounts for a small but meaningful fraction of cancer diagnoses.
Why does the primary site sometimes stay hidden? The original tumor may be tiny, may have been destroyed by the immune system after seeding its metastases, or may be located in a spot that routine imaging misses. In recent years, molecular profiling, which analyzes the genetic and protein signatures of the metastatic tumor, has helped narrow the likely origin in many CUP cases. Knowing the probable origin matters because treatments designed for a specific carcinoma type tend to work better than generic chemotherapy, even when the primary tumor itself is never found.
What Causes Carcinomas
The vast majority of carcinomas are not inherited. Research suggests that roughly 93 percent of human cancers result from interactions with environmental and lifestyle factors rather than inherited genetic mutations.13PubMed Central. Environmental Factors Inducing Human Cancers That does not mean genes are irrelevant. Cancer develops when mutations accumulate in specific types of genes, including genes that normally suppress tumor growth, genes that promote cell division, genes responsible for repairing DNA errors, and genes that control programmed cell death.14PubMed Central. Environmental Factors Inducing Human Cancers Most of those mutations are acquired over a lifetime through exposure to carcinogens, chronic inflammation, or simply the accumulated errors of normal cell division.
The specific environmental triggers vary by carcinoma type. Ultraviolet radiation drives skin carcinomas. Tobacco smoke is the dominant cause of squamous cell and small cell carcinomas of the lung. Chronic infection with certain viruses, such as HPV for cervical carcinoma and hepatitis B or C for liver carcinoma, plays a central role in some populations. Alcohol, obesity, and certain occupational chemical exposures round out the major risk categories. The common thread is that epithelial cells, sitting on the body’s surfaces and organ linings, are the first line of contact with many of these damaging agents.
Aggressiveness Varies Widely
Not all carcinomas behave the same way. Basal cell carcinoma of the skin almost never spreads to distant organs and is rarely life-threatening. At the other extreme, pancreatic adenocarcinoma is one of the deadliest cancers because it often goes undetected until it has already spread widely. Between these poles sits a vast spectrum.
Even within a single carcinoma category, behavior can vary dramatically depending on microscopic features. Among cutaneous squamous cell carcinomas, for example, the common invasive type is the most frequently encountered and has a relatively predictable course. But certain subtypes carry elevated risk: poorly differentiated spindle cell variants can grow rapidly and behave aggressively, while desmoplastic squamous cell carcinomas, especially thick ones, carry a high risk of recurrence and spread.15PubMed. Histologic subtyping and malignancy assessment of cutaneous squamous cell carcinoma By contrast, verrucous carcinomas, another squamous cell variant, are categorized as low-malignancy growths.16PubMed. Histologic subtyping and malignancy assessment of cutaneous squamous cell carcinoma
This is why a pathology report goes far beyond simply saying “carcinoma.” The report will typically describe the histological subtype, the degree of differentiation (how much the tumor cells still resemble normal tissue), tumor thickness or depth of invasion, whether there is evidence of involvement of lymphatic or vascular channels, and the status of surgical margins. All of these details feed into the staging system and influence what treatment is recommended.
Carcinoma and Age
Carcinomas are overwhelmingly diseases of adulthood and older age. The accumulation of DNA damage over decades is a major reason. Children and adolescents do develop carcinomas, but these are considered rare in pediatric oncology. When they do occur in younger populations, they tend to involve specific organs: thyroid carcinoma, nasopharyngeal carcinoma, adrenocortical carcinoma, and colorectal carcinoma have all been recognized as uncommon but notable diagnoses in children and young adults.17PubMed Central. Rare Tumors: Opportunities and challenges from the Children’s Oncology Group perspective The rarity of carcinomas in children highlights how much cumulative environmental exposure and time-dependent mutation accumulation matter in the development of these cancers.
In older adults, carcinomas dominate the cancer landscape. Lung, breast, prostate, and colorectal carcinomas are the most commonly diagnosed cancers in many countries, and all four are carcinomas. The rising risk with age is one reason that screening programs for breast, colorectal, cervical, and lung cancer are targeted at middle-aged and older populations rather than the young.
Treatment and the Shift Toward Precision Approaches
Treatment for carcinoma depends on the type, stage, and molecular characteristics of the tumor. Surgery remains the mainstay for most localized carcinomas: removing the tumor and a margin of healthy tissue is curative in many early-stage cases. Radiation therapy, chemotherapy, and hormone therapy (for hormone-driven carcinomas like many breast and prostate cancers) have long formed the core toolkit.
What has changed dramatically in recent years is the growing role of immunotherapy and targeted therapy. In metastatic renal cell carcinoma, for instance, immunotherapy combinations are now the standard first-line treatment.18PubMed Central. Evaluation of tumor microenvironment and biomarkers of immune checkpoint inhibitor response in metastatic renal cell carcinoma These drugs work by helping the immune system recognize and attack cancer cells. Researchers are actively working to identify gene signatures and immune markers that predict which patients will respond well and which will not, since response rates vary significantly from person to person.19Cancer Research. Abstract A009: Multicellular modules and tumor programs shape the tumor microenvironment and predict immune checkpoint inhibitor (ICI) response in clear cell renal cell carcinoma (ccRCC)
Similar precision approaches are expanding across carcinoma types. Lung adenocarcinomas, for example, are now routinely tested for specific genetic mutations that can be targeted with drugs designed to block those exact pathways. Breast carcinomas are classified not just by their microscopic appearance but by their molecular profile, which determines whether they are treated with hormone-blocking drugs, targeted antibodies, or chemotherapy. The name “carcinoma” is the starting point, but treatment increasingly depends on the molecular fingerprint of the individual tumor.
Carcinomas in Other Species
Carcinomas are not unique to humans. Mammary carcinomas occur across the animal kingdom, and comparing the disease in domestic animals with breast cancer in people has become a productive area of research. Dogs and cats both develop spontaneous mammary cancers, and the most common forms, including tubulopapillary, solid, and cribriform carcinomas, share meaningful similarities with human breast cancer.20PubMed. Comparative pathology of mammary gland cancers in domestic and wild animals Researchers in comparative oncology view spontaneous mammary cancers in cats and dogs as strong models for studying human breast cancer, since these animals develop the disease naturally rather than having it artificially induced in a lab.
This cross-species perspective is useful because it helps untangle which aspects of carcinoma biology are fundamental to epithelial cells in general and which are specific to human risk factors like smoking, alcohol, or industrial chemical exposure. If a particular molecular pathway drives carcinoma in dogs, cats, and humans alike, it points toward deep evolutionary vulnerabilities in epithelial tissue itself rather than anything peculiar to human lifestyle.