Cancer isn’t a single disease. It’s a group of more than 100 different diseases, each named for the type of cell or tissue where it starts. The five major categories are carcinomas, sarcomas, leukemias, lymphomas, and myelomas. Globally, an estimated 20 million new cancer cases were diagnosed in 2025, with lung, breast, colorectal, prostate, and non-melanoma skin cancers being the five most common.
Carcinomas: The Most Common Category
Carcinomas account for the vast majority of cancer diagnoses. They start in epithelial cells, which are the cells that line your skin, organs, and internal passageways. Because epithelial tissue covers so much of the body, carcinomas can appear in dozens of locations. There are three main subtypes worth knowing.
Adenocarcinoma begins in glandular cells that secrete fluids like mucus or digestive juices. Most prostate, breast, colorectal, and pancreatic cancers are adenocarcinomas. It’s also responsible for about 85% of all kidney cancers and is the most common form of liver cancer.
Squamous cell carcinoma starts in the flat cells that make up the outermost layer of your skin and line areas like the lungs, esophagus, and throat. On the skin, it tends to appear in sun-exposed areas: face, ears, neck, arms, and legs.
Basal cell carcinoma develops in the layer of cells that sits beneath the squamous cells in your skin. It’s the most common type of skin cancer overall, though it rarely spreads to distant parts of the body.
Sarcomas: Bone and Soft Tissue
Sarcomas are rare cancers that arise from connective tissues, the structural materials that hold the body together. They fall into two broad groups: bone sarcomas and soft tissue sarcomas.
The most common bone sarcomas are osteosarcoma (which produces abnormal bone tissue), Ewing sarcoma (most often diagnosed in children and young adults), and chondrosarcoma (which forms in cartilage). Soft tissue sarcomas develop in fat, muscle, blood vessels, or fibrous tissue. Liposarcoma, which starts in fat cells, is among the most frequently diagnosed.
Because sarcomas are uncommon, they are often harder to recognize early. They can form almost anywhere in the body, and a painless lump that grows over weeks or months is a typical first sign.
Blood Cancers: Leukemia, Lymphoma, and Myeloma
Unlike carcinomas and sarcomas, blood cancers don’t form solid tumors. They affect the cells of the blood, bone marrow, and lymphatic system, which is why they’re sometimes called “liquid” cancers. The three main types each target different cell populations.
Leukemia is a group of cancers affecting white blood cells. Abnormal white blood cells multiply in the bone marrow and crowd out healthy cells, leading to problems like fatigue, frequent infections, and easy bruising. Leukemia can be acute (fast-growing) or chronic (slow-growing).
Lymphoma is cancer of the lymphatic system, which includes your lymph nodes, spleen, and bone marrow. It comes in two broad forms: Hodgkin lymphoma and non-Hodgkin lymphoma. Non-Hodgkin lymphoma is far more common and includes dozens of subtypes.
Multiple myeloma targets plasma cells, a type of white blood cell that normally produces antibodies to fight infection. When plasma cells become cancerous, they accumulate in the bone marrow and can weaken bones, suppress the immune system, and damage the kidneys.
How Cancer Is Named When It Spreads
Cancer that spreads from its original site to another part of the body is called metastatic cancer. One point that often confuses people: a cancer always keeps the name of the tissue where it started. Breast cancer that spreads to the lung is called metastatic breast cancer, not lung cancer. It’s treated as stage 4 breast cancer because the cells still behave like breast cancer cells, even in their new location.
Cancer cells spread by breaking away from the original tumor and traveling through the bloodstream or the lymphatic system. Once they settle in a new organ or tissue, they can form new tumors there. This ability to invade and spread is the defining feature that separates a malignant (cancerous) tumor from a benign (noncancerous) one. Benign tumors stay localized and don’t invade surrounding tissue.
How Cancer Is Staged
Staging describes how far a cancer has progressed in the body. Most solid tumors use the TNM system, which evaluates three factors. T refers to the size of the primary tumor, with higher numbers (T1 through T4) indicating a larger tumor or more growth into surrounding tissue. N describes whether cancer has reached nearby lymph nodes, and if so, how many. M indicates whether the cancer has metastasized to distant parts of the body: M0 means it has not, M1 means it has.
These three factors combine to give an overall stage, typically expressed as stage 1 through stage 4. A stage 1 cancer is small and confined to where it started. Stage 4 means it has spread to distant organs. Staging matters because it shapes treatment decisions and helps predict outcomes.
Rare Cancers
A cancer is classified as rare when it affects fewer than 40,000 people per year in the United States. That threshold encompasses a surprisingly large number of cancer types, including rare bone tumors, rare kidney tumors, rare endocrine tumors, and rare vascular tumors. All childhood cancers are considered rare simply because cancer rates in children are very low overall.
Rare cancers can be harder to diagnose because many doctors encounter them infrequently. They also tend to have fewer established treatment protocols and fewer clinical trials, which can make finding specialized care more challenging.
Benign vs. Cancerous Tumors
Not every tumor is cancer. Benign tumors grow in one place and don’t invade nearby tissue or spread. They can still cause problems if they press on nerves, blood vessels, or organs, but they aren’t inherently life-threatening in the way cancerous tumors are.
Cancerous tumors, by contrast, can grow into surrounding tissue, break through boundaries between cell layers, and seed new tumors in distant organs. On a physical exam, cancerous lumps tend to feel firm and less movable than benign ones, though imaging and biopsy are needed for a definitive answer. A biopsy, where a small sample of tissue is examined under a microscope, remains the gold standard for distinguishing benign from malignant growths.