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What is cancer?

A cancer patient and her doctor sitting at a table as they go over information on a tablet.
A cancer patient and her doctor sitting at a table as they go over information on a tablet. (Image credit: Getty Images)

Cancer is a difficult and often misunderstood condition. A diagnosis of cancer might feel like a death sentence, but the truth can be much more complicated. 

Cancer is an umbrella term for any uncontrolled growth (sometimes called a tumor) in the body that spreads out from where it started. This spreading characteristic makes the tumor malignant, which means it's more deadly. However, if the growth stays in one place, it's considered benign, which means it's not cancerous and much less likely to be harmful or dangerous. 

"A cancer cell was at one point a normal part of your body. Somewhere along the course, it lost its ability to function like a normal cell," said Dr. Ishwaria Subbiah, a palliative care physician and medical oncologist at The University of Texas MD Anderson Cancer Center. "What makes a cell a cancer is this loss of self-regulation, to where now it grows uncontrollably and spreads to different parts of the body."

Many cancer cases can be treated effectively, depending on the type and how early doctors find it. But as cancer spreads, it may take over the body's essential organs — called metastatic cancer — eventually killing the patient. This is why immediately notifying a doctor of any unusual symptoms or changes in the body and doing recommended cancer screenings are essential to catching cancers early.

What are the symptoms of cancer?

Some cancers, especially those in the organs near the surface of the body, can be detected by touch — feeling for lumps or bumps. For example, monthly breast self-screening for lumps is an important way to detect breast cancers early.  

But, depending on where cancer arises, how large the growth or tumor is, and if the cancer has spread, some cancer patients may feel no symptoms at all for quite some time. 

Related: Breast cancer: The first sign isn't always a lump

"Some of the most difficult cancers to treat are partly because by the time someone feels symptoms from them, they've grown pretty large or they've spread to other parts of the body," Subbiah told Live Science. "You really are very unlikely to feel it until it maybe gets bigger and starts pushing on some of the ducts and the other organs."

Some early cancer symptoms that a person may feel before being diagnosed include a general feeling of exhaustion or tiredness beyond normal, losing weight without actively trying and symptoms including night sweats, according to the American Cancer Society.

"There are many illnesses in the human body that manifest with these vague symptoms," Subbiah said. "If someone's having night sweats or they're feeling fatigue, they shouldn't automatically think of cancer. But it [should be] a trigger for them to see their primary care physician."

How is cancer diagnosed?

If a patient has symptoms that a doctor suspects may signal cancer, they usually need to do a few tests before getting a cancer diagnosis, according to the National Cancer Institute. First, the doctor will ask the patient for an extensive family and personal history to check for any risk factors for cancer. The doctor may also order tests to analyze the patient's blood and look for signs of cancer.

Then, the doctor will move on to imaging tests — CT, MRI or other types of scans that look for abnormal growths. These scans can't usually tell the doctor if the tumor is malignant, but they can show the size of it, if it's impacting any major organs, if it looks like it has spread beyond the tissue it came from and if there are multiple growths in more than one organ. 

"Almost always, the cancer looks different from the normal tissue that surrounds it," Subbiah said. Depending on the type of imaging used, cancer may appear as a bright spot or a dark spot compared to normal tissue, she said. 

If there is a growth seen on an imaging test, the doctor will likely want to examine a piece of it in the lab. The process of taking a small piece of an unusual growth to examine it under a microscope is called a biopsy. It may be as simple as removing a small circle of skin, or it may require surgery to access the tumor and take a sample. 

That sample is then studied by a pathologist, which is a doctor who specializes in analyzing tissue samples. First, the pathologist takes fine slices of the tissue and stains them with chemicals that highlight different types of cells and structures within the cells. Then, the pathologist can analyze how the cells look, what tissues they're from and several other features of the cells. Specifically, they can see how quickly cancer grows by counting the number of cells in the cancerous tissue that are dividing. 

"If we see most or all of the cells are actively dividing, that suggests more aggressive behavior," Subbiah said. "These are all bits of information we would get from seeing [the cancer] under the microscope that will help us understand that individual person's cancer on a much deeper level."

The pathologist's findings can also tell the doctor more about where the cancer started. 

How does cancer start?

A cancer cell, as defined by the National Cancer Institute, is any cell that goes through changes to its genetic material that enable it to evade the body's natural defenses against uncontrolled growth. 

All cancers are caused by changes to the normal cells' genetic code, or DNA. This code tells cells what to do and when. It includes instructions for how to build all of the active and structural proteins that give the body's cells their shape and determine how they carry out their daily tasks. 

Every time cells divide, they make a copy of the genetic code contained within them. This copying process isn't always perfect and occasionally results in typos, or errors in the DNA, which are called mutations. Cells have ways to detect and fix these mistakes, but they don't always work. If a cell develops a mutation, it may start to act differently to a normal cell.

Related: 'Doughnut-shaped' DNA makes cancer more aggressive

"The cells of our body develop new mutations all the time, [and] your natural mechanisms pick up on that mutation, and they decide whether that cell can continue that way or that cell needs to die," Subbiah said. Tumors become malignant, or cancerous, when they develop these mutations that let them escape the body’s natural mechanisms to defeat unruly cells.

There are typically three evasive moves a cell can take to become cancerous. Either a mutation lets the cell divide uncontrollably, helps it escape the immune system's normal cell-repair systems or makes it resistant to the body's processes for getting rid of cells that are getting older and their DNA is breaking down and developing mutations. Many cancers acquire mutations that enable more than one of these evasive moves. 

What causes cancer?

While the potential causes of some cancers are well established, such as smoking causing lung cancer, ultraviolet radiation exposure leading to skin cancers, and human papillomavirus (HPV) causing cervical cancer, most cancers don't have a direct cause. It's mostly just by chance that some of the mutations that occur affect how DNA controls cancer-related properties of the cell, Subbiah said, and it's by chance that those cells keep growing and making new typos to develop into cancer cells.

When there are specific causes for a cancer, it can usually be traced back to exposure to something in the environment that causes more mutations than usual. According to the American Cancer Society, these environmental triggers include smoking, infections like HPV and exposure to radiation, including UV light, like sunlight. A person's diet and physical activity can also increase the body's chances of developing mutations, potentially influencing the risk of developing cancer. 

Related: Here's how many US cancer cases are tied to unhealthy diets

For some people, genetic syndromes traced to broken parts of a person's DNA play a role in their cancer risk, but most cancers aren’t due to specific inherited genes.

How is cancer treated?

As researchers have learned more about the causes of cancer over the last several decades, cancer treatment has gone through a total transformation. "The pillars of cancer treatment were stable for about 50 or so years," Subbiah said. "In the last 20 years, they've completely changed."

Traditional cancer treatments include surgery, radiation therapy and chemotherapy. These tools have been used for decades to treat cancer and are continually being improved.

Surgery physically removes cancer from the body. A doctor cuts into the body, finds the cancerous growths and removes them. Surgery techniques are improving, becoming less invasive and leaving fewer long-term effects, Subbiah said. 

Because surgery can leave behind microscopic pieces of cancer or miss cancer cells that have already spread to other tissues or organs, doctors usually recommend other treatments as well, according to the National Cancer Institute. For example, a patient may have surgery to remove a tumor, then receive radiation treatment or chemotherapy to kill any cancer cells that remain.

Related: Cancer patients weren't responding to therapy. Then they got a poop transplant.

Radiation therapy uses high-energy waves to physically destroy cancer cells. The radiation can either be administered with a beam from outside the body or from inside the body using an implant, according to the National Cancer Institute. Radiation waves damage the cancer cells' genes, eventually killing them. 

Chemotherapy destroys the body's rapidly growing cells. Cancer cells grow and divide quickly, but so do some of the body's normal tissues. Chemotherapy's characteristic side effects are due to the destruction of these healthy cells — those of the hair follicles, the lining of the mouth and the intestines, according to the National Cancer Institute

"One hundred percent of people who go through cancer treatment will have symptoms — either from the cancer itself or the treatment," Subbiah said.

Treatments can target cancer symptoms, side effects from other treatments and the mental effects of a cancer diagnosis. "For some people, the medications can make the symptom go away altogether. And for others, it can bring it down to a much, much more tolerable level," Subbiah said. In addition, treatment for many of the emotional symptoms that come with cancer is important. "We pay equal attention to the mental health and wellbeing of that person."

Cancer is a "very, very intelligent enemy; it's an evolving enemy and it's one that adapts," Subbiah said, and sometimes it outsmarts the available treatments against it. When the cancer is no longer kept in check by treatments, it can become deadly. 

The uncontrolled growths push the normal, healthy cells out and take over organs that are essential for survival. Lung tumors may interfere with the patient's ability to breathe; liver cancer may cause liver failure; or brain damage can interrupt the body's normal functions, Subbiah said. 

Recent advancements and future directions

Over the last 20 or so years, researchers have added many new tools to the cancer-fighting arsenal. The biggest leaps in cancer research have recently focused on new treatments that target cancer directly and trigger the immune system to fight it off. 

Targeted therapies are drugs designed to home in on cancer cells based on their unique characteristics. These may include abnormal proteins produced by mutations that have let them grow uncontrollably or evade the immune system, according to the American Cancer Society

"There's much less collateral damage" with targeted therapies compared with older cancer drugs, Subbiah said. "The vast majority of the cells in your body don't have that [cancerous] mutation, so they're not going to be damaged by the targeted therapies."

Immunotherapies are another class of drugs, which function to release the natural brakes on the body's immune system, strengthening its natural ability to attack cancer, according to the American Cancer Society. While there are side effects to immunotherapies, they're usually milder than those caused by traditional chemotherapy. 

In addition to next-generation traditional therapies, new gene and cellular therapies are also on the horizon, Subbiah said. 

Cellular therapies, such as stem cell transplants, have been used for decades to treat blood cancers, according to a 2019 editorial in the journal Frontiers in Immunology. Right now, doctors can remove a patient's blood cells, treat them to remove the cells with cancer-causing gene mutations and inject these healthy cells back into the body. 

In the future, cell-based therapies may treat solid cancers by sending modified cells into the body that can target cancer-related proteins and fight and kill the cancer cells, Subbiah said.

Another promising approach to cancer treatment is gene-based therapies, as described in a 2015 review in the Journal of Cellular Physiology. Gene editing could potentially fix mutations that make cells cancerous, or make cancerous cells easier for the immune system to identify and destroy. These types of treatments could be ready for widespread application in the next 10 or so years, Subbiah said.

"Cancer is one of the few fields in medicine that if you pick up a book on how to treat cancer, you have to be sure it's from the past two to three years maximum," she said. "Anything beyond that is completely outdated."

Additional resources

This article is for informational purposes only, and is not meant to offer medical advice. 

Jennifer Welsh

Jennifer Welsh is a Connecticut-based science writer and editor with several years of bench work in cancer research and anti-viral drug discovery under her belt. She has previously written for Science News, VerywellHealth, The Scientist, Discover Magazine, WIRED Science, and Business Insider.