Cancer is not one disease—it's more than 200 different diseases

Doctors have identified more than 200 types of cancer, and the number continues to grow as medical science gets better at spotting differences between tumors. Each type starts in a different cell or tissue, behaves differently, spreads at different speeds, and responds to different treatments. Two people with "cancer" may have almost nothing in common medically, even though they use the same word to describe their diagnosis.

The main way doctors sort cancers is by where they start: skin cancer starts in skin cells, lung cancer in lung cells, breast cancer in breast tissue. But even within one location, there can be many subtypes. Lung cancer in a smoker who never smoked looks different under a microscope than lung cancer in someone who never smoked. Those differences matter for treatment.

Key Takeaways

  • Cancers are named by the tissue or organ where they begin, and more than 200 types have been identified so far.
  • Two cancers in the same organ can behave very differently depending on what kind of cells they started in and how fast they grow.
  • Doctors use the cell type, stage, and genetic makeup of a tumor to decide on treatment, not just the location.
  • Common cancers like breast, lung, prostate, and colorectal account for a large share of diagnoses, but rare cancers exist in nearly every tissue in the body.

How cancers are organized and named

A cancer's name comes from the tissue or organ where it starts, not where it spreads. Breast cancer that spreads to the bones is still breast cancer, not bone cancer. This matters because the original cell type determines how the cancer will likely behave and which drugs will work against it.

Doctors also sort cancers by the type of cell they come from. Carcinomas start in cells that line organs or glands—these include most breast, lung, colon, and skin cancers. Sarcomas start in bone, muscle, or connective tissue. Leukemias start in blood-forming cells in the bone marrow. Lymphomas start in immune system cells. Melanomas are a specific type of skin cancer. Within each category, there are many subtypes based on exactly which cells are involved.

The most common cancers and why they matter

A handful of cancers account for most diagnoses. Breast cancer, lung cancer, prostate cancer, and colorectal cancer together make up roughly half of all new cancer cases in the United States. Because they are common, there is more research funding, more treatment options, and more doctors with experience treating them.

Rarer cancers—like mesothelioma, which starts in the lining around the lungs, or cholangiocarcinoma, which starts in bile ducts—get less research attention and fewer treatment centers. People with rare cancers sometimes have to travel to find a doctor who has treated their specific type before. The National Institutes of Health and cancer organizations maintain registries of rare cancers partly to help patients find specialists.

Why the same cancer type can behave very differently

Two people diagnosed with breast cancer might receive completely different treatment plans. One might have a slow-growing tumor that responds well to hormone therapy. The other might have an aggressive tumor that needs chemotherapy and radiation. The difference comes down to what the cancer cells look like under a microscope, how fast they are dividing, and what genetic mutations they carry.

Modern pathology reports now include information about the tumor's genetic profile—which genes are mutated, which proteins are present on the cell surface. A tumor with a HER2 mutation in breast cancer responds to drugs that target that specific mutation. A tumor without it does not. This is why two people with the same cancer type can need very different drugs.

Staging and grade: two other ways cancers are classified

Beyond type, doctors use stage and grade to describe a cancer. Stage refers to how far the cancer has spread: stage 1 is localized, stage 4 has spread to distant organs. Grade refers to how abnormal the cells look and how fast they are dividing—a grade 1 tumor looks more like normal cells and grows slowly, while a grade 3 tumor looks very abnormal and grows fast.

A stage 1, grade 1 breast cancer and a stage 4, grade 3 breast cancer are the same type of cancer but require very different treatment intensity and have different outlooks. Staging and grading help doctors and patients understand what they are dealing with and what to expect.

Cancers that start in different places but are treated the same way

Sometimes cancers that start in different organs respond to the same drug because they share a genetic mutation. A lung cancer and a melanoma might both have a BRAF mutation, and both might respond to a drug that targets BRAF. This is why genetic testing of the tumor itself—not just knowing where it started—has become central to cancer treatment.

This shift means that a cancer's location is no longer the only thing that matters. A tumor's genetic profile can matter more. A drug approved for one type of cancer might be used off-label for a different type if both tumors have the same mutation. This has opened up treatment options that would not have existed 10 or 15 years ago.

Subtypes within common cancers

Even within a single cancer type, there are many subtypes. Lung cancer divides into small cell and non-small cell types, and non-small cell lung cancer further divides into adenocarcinoma, squamous cell carcinoma, and large cell carcinoma. Each subtype has different risk factors, different typical locations within the lung, and different treatment responses.

Breast cancer subtypes include luminal A, luminal B, HER2-positive, and triple-negative, based on which receptors and proteins are present on the cancer cells. These subtypes predict which treatments will work and how aggressive the cancer is likely to be. A pathology report for breast cancer will usually include this subtype information because it directly shapes the treatment plan.

Frequently Asked Questions

Is there a cancer type that is most common?

Breast cancer is the most common cancer overall, followed by lung cancer and prostate cancer. In men, prostate cancer is most common; in women, breast cancer is. Colorectal cancer is also very common in both sexes. These four cancers account for a large share of all new diagnoses.

Can a person have two different types of cancer at the same time?

Yes. A person can develop a second cancer that is unrelated to the first. This can happen years later or sometimes at the same time. The second cancer is treated as a separate diagnosis with its own stage, grade, and treatment plan.

Why do doctors do genetic testing on tumors?

Genetic testing reveals which mutations the cancer cells carry. This information tells doctors which drugs are most likely to work and which ones probably will not. A drug that targets a specific mutation only helps if the tumor has that mutation, so testing prevents unnecessary treatment and guides doctors toward the most effective options.

Are new cancer types still being discovered?

New subtypes and variants are identified as technology improves, but the major cancer types have been known for decades. What changes is the level of detail doctors can see—better microscopes and genetic sequencing reveal differences that were invisible before, leading to more precise classification and treatment.

Does the type of cancer determine the treatment?

The type is one factor, but not the only one. Stage, grade, genetic mutations, and the patient's overall health all shape the treatment plan. Two people with the same cancer type might receive different treatments based on these other factors.