There are more than 100 types of cancer, according to the National Cancer Institute. That number grows substantially when you factor in molecular subtypes, which modern genetic testing can now identify within a single cancer type. The World Health Organization maintains a multi-volume classification system, now in its 5th and 6th editions depending on the body site, that catalogs hundreds of distinct tumor types across 15 organ systems.
Why the Count Depends on How You Count
The “more than 100” figure refers to cancers grouped by where they start in the body: lung cancer, breast cancer, colon cancer, and so on. But within each of those, there are subtypes based on how the cells look under a microscope and, increasingly, on their genetic makeup. Breast cancer alone has at least six recognized molecular subtypes, each with different survival rates and treatment responses. Multiply that kind of granularity across every organ system, and the true number of distinct diseases classified as “cancer” reaches into the hundreds.
The WHO’s tumor classification books span 15 volumes covering everything from brain tumors to skin tumors to blood cancers, each volume listing dozens of individual tumor types. These aren’t academic curiosities. The specific subtype determines which treatments work, how aggressive the disease is, and what the outlook looks like.
The Five Major Categories
Every cancer falls into one of a handful of broad categories based on the type of tissue where it begins.
- Carcinoma starts in the skin or in tissues that line or cover internal organs. This is the most common category and includes subtypes like adenocarcinoma (glandular tissue), squamous cell carcinoma (flat surface cells), and basal cell carcinoma (deeper skin cells). Most lung, breast, colon, and prostate cancers are carcinomas.
- Sarcoma begins in connective or supportive tissues: bone, cartilage, fat, muscle, or blood vessels. These are far less common than carcinomas.
- Leukemia is cancer of the white blood cells. It starts in blood-forming tissues like bone marrow and results in large numbers of abnormal blood cells entering the bloodstream.
- Lymphoma starts in the lymph glands or other parts of the lymphatic system, the network that helps your body fight infection. It occurs when certain immune cells begin dividing abnormally.
- Myeloma starts in plasma cells, a type of white blood cell in bone marrow that normally produces antibodies to fight infection.
There are also cancers that don’t fit neatly into these five groups, including brain and spinal cord tumors and certain germ cell tumors.
Molecular Subtypes Changed the Picture
For decades, cancers were classified purely by how cells looked under a microscope. That system worked, but it had a major blind spot: tumors that looked identical could behave very differently in different patients. Gene expression analysis changed that. By examining which genes are active in a tumor, researchers can now split what was once considered a single disease into subtypes with meaningfully different outcomes.
Breast cancer is a clear example. Traditional classification relied on whether tumor cells had receptors for estrogen, progesterone, and a protein called HER2. Molecular profiling revealed that the estrogen-receptor-positive group actually contains at least two distinct subtypes, Luminal A and Luminal B, with very different survival rates. A 50-gene test can now sort breast tumors into molecular subtypes using a routine lab procedure, guiding whether a patient needs chemotherapy or can safely skip it.
This kind of molecular reclassification is happening across cancer types, from lung cancer to leukemia. It means two patients with “the same cancer” may have biologically different diseases requiring different treatments.
The Most Common Types
A small number of cancers account for the majority of diagnoses. In the United States, projected new cases for 2026 show prostate cancer leading at roughly 334,000 cases, followed closely by breast cancer at about 325,000. Lung cancer comes next at around 229,000, then colorectal cancer at nearly 159,000. Melanoma rounds out the top five at 112,000 new cases.
The next tier includes bladder cancer (about 85,000 cases), kidney cancer (80,000), non-Hodgkin lymphoma (79,000), uterine cancer (68,000), and leukemia (68,000). Together, these ten types represent the bulk of cancer diagnoses, even though they make up only a fraction of the 100-plus recognized types.
Most Cancer Types Are Actually Rare
Any cancer that affects fewer than 40,000 people per year in the U.S. is classified as rare. By that definition, the vast majority of the 100-plus cancer types qualify. All childhood cancers are considered rare simply because cancer rates in children are very low overall.
Rare cancers present a particular challenge. Fewer patients means fewer clinical trials, less research funding, and fewer oncologists with deep experience treating them. A patient diagnosed with a rare sarcoma subtype faces a very different landscape than someone diagnosed with a common form of breast or prostate cancer, even though both fall under the same broad umbrella of “cancer.”
How Cancers Get Their Names
Cancer names typically combine the tissue of origin with the category. An adenocarcinoma of the lung started in glandular tissue inside the lung. An osteosarcoma is a sarcoma of the bone (“osteo” meaning bone). A hepatocellular carcinoma is a carcinoma that began in liver cells (“hepato” meaning liver). Some cancers carry historical names, like Hodgkin lymphoma, named after the physician who first described it, but the naming logic is otherwise consistent: where it started plus what type of tissue is involved.
This naming system matters practically because a cancer is always defined by where it originated, not where it spreads. Breast cancer that has spread to the lungs is still breast cancer, not lung cancer, and is treated with breast cancer therapies. The distinction affects everything from treatment selection to clinical trial eligibility.