Cancer occurs when some of the cells in the body start growing uncontrollably. It can start almost anywhere and spread to other body systems. Mutations in cancer cells cause them to behave abnormally, affecting the disease's development, progression, and spread. These mutations can also contribute to how well the disease responds to treatment and whether or not the cancer comes back.

There are more than a hundred million cells in the body; cancer can start with just a small change in one of them.

Tumor Initiation

Most cancers start with a primary tumor, though some, like leukemia, begin in the blood or bone marrow and do not form a solid mass.

Every cell in the body does a different job, but they are all made up of the same parts. The nucleus is the control center and contains the cell's genetic information. These genes tell the cells what to do, determining not only what type the cell will be but also what it does, when it divides, and when it dies. Cancer begins when a change in one of these genes causes a single cell to reproduce abnormally.

Doctor in conversation with patient Solskin / Getty Images


Tumor Progression

In some cancers, as the abnormal cell reproduces, it forms a mass called a tumor. The tumor progresses as more mutations occur that give some of these abnormal cells an advantage, allowing them to spread faster. These genetic changes continue as the malignant tumor grows, helping it become more efficient: it grows faster and becomes increasingly malignant.

Molecular Biologist hold blood sample for KRAS mutation test for lung cancer. Md Saiful Islam Khan / Getty Images


How Cancer Cells Are Different

Many things make cancer cells different from normal cells and allow them to multiply and spread. For example, normal cells only grow when they receive signals from the body; cancer cells grow without them. The body also tells normal cells when to stop dividing or die; cancer cells do not respond to these signals.

Normal cells stop growing when they encounter other cells, but cancer cells invade this other tissue. Cancer cells can also hide from the immune system, which would usually attack abnormal cells. Some cancerous cells have twice the number of chromosomes, accumulating more mutations to become more efficient. They may also rely on different nutrients for energy, allowing them to grow more quickly.

breast cancer concept PonyWang / Getty Images


How Cancer Cells Start to Spread

Cancer cells spread using various methods. Generally, malignant cells secrete enzymes that digest some of the structural components of neighboring healthy cells, allowing the cancer cell to invade. As the tumor grows, it needs a larger blood supply, so malignant cells secrete growth factors to promote the creation of more blood vessels.

In addition to providing the tumor with the nutrients it needs to grow larger, an extensive blood supply also gives the cancer cells a way to enter the bloodstream and spread to other parts of the body.

x-ray lung cancer utah778 / Getty Images



When cancer spreads, it is called metastasis. Malignant cells follow a few steps to do this. They may invade nearby tissue or move into blood or lymph vessels to travel to other parts of the body. Cancer cells that end up in a distant location then move through the blood vessel walls and into the surrounding tissue, where they multiply, forming another tumor.

The cancer cells then secrete growth factors to establish a new blood supply, and the metastatic tumor continues to grow. Sometimes, metastatic cells remain inactive for years before they begin to grow, or they may not grow at all.

Breast cancer, damaged cells, Female breast anatomy, 3d illustration Mohammed Haneefa Nizamudeen / Getty Images


Where Cancer Spreads

Cancer can spread anywhere, but some types are more likely to spread to certain areas than others. For example, breast cancer is more likely to spread to the brain, bone, lung, or liver; lung cancer to the adrenal glands, brain, liver, bone, or opposite lung; melanoma to the lung, skin, brain, bone, or muscle; and ovarian cancer to the lung, liver, or peritoneum.

Male doctor in surgical clothes looking at vertebral mri scan megaflopp / Getty Images


Cancer Staging

Cancer staging determines how much the cancer has spread. One method doctors use to stage cancer is the TNM system, which is divided into five stages, from zero to four. Stage zero, also called in situ, is when abnormal cells are present but have not yet spread; this stage is not cancer, but it can become cancer and requires treatment.

In stages one to three, cancer is present; the higher the number, the more it has spread. You may also hear your doctor use terms like localized, which means that the cancer has not spread; regional, when the cancer has spread to nearly lymph nodes, organs, or tissues; or distant, when the cancer has spread to other parts of the body.

Doctor talking with patient at desk in medical office Lordn / Getty Images


Why Cancer Might Come Back After Treatment

Cancer may come back after treatment for many reasons. There are multiple ways to fight cancer: surgery, radiation, chemotherapy, and immunotherapy. Regardless of the type, if the treatment does not kill all of the cancer cells, the tumor can regrow, or the cells can spread.

Metastatic cancer cells can also remain in the body for years before becoming a tumor.

Oncology patient in the hospital FatCamera / Getty Images


Treatment Resistance

Because of their ability to mutate and change, some cancers can resist treatment. Cancer cells can become resistant to chemotherapy, targeted therapy, or multiple drugs. Research has found that specific mutations enable cancer cells to keep the drugs out by creating high levels of a specific protein that acts as a pump, removing toxins from the cells.

African American man pouring medications out of a bottle into his hand. Lock Stock / Getty Images



Doctors may not be able to tell someone with cancer that they are cured, which can be challenging to deal with on a daily basis. Generally, each day post-treatment without a recurrence lowers the risk of cancer coming back. Many cancers that are going to return do so within two years. Recurrence is even less likely five years post-treatment, and doctors may say the cancer is cured after ten years.

Hispanic female expertise giving advice to cancer patient while examining x-ray at doctor's office Maskot / Getty Images


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