Читать книгу Overcome the Challenges of Cancer Care - M. D. Rosenberg - Страница 5

Chapter 1

Cancer 101

This chapter provides a broad overview of what cancer is, including where it comes from and how to think about it. Cancer is just a shadow of us. To get to know cancer, we need to get to know ourselves.

Cancer: Millions of Diseases

Cancer is not one disease. It’s millions of them. It varies from person to person and even within the same person. We see a huge range in prognosis (the course a disease will take) and patients’ survival times.

Suppose a woman named Leslie develops a breast cancer, and so does her neighbor Jill. The biology (behavior) of Leslie’s cancer is going to be different from Jill’s. Leslie’s cancer may respond to chemotherapy, while Jill’s cancer may not. Such differences make it hard to come up with a single cure for cancer.

Overall, cancer death rates have declined, and the number of cancer survivors has increased. This is great news! But it does mean we need to look at the long-term effects of cancer and its treatment.

Where in the Body Does Cancer Come From?

Cancer comes from almost anywhere in the body. No organ or cell is immune from turning into cancer.

Cancer, at its core, comes from cells, which are the building blocks of your body. The average human body is made up of 30 to 40 trillion cells. Each cell has a nucleus, which you can think of as the control center or brain of the cell.

Stored in the nucleus is the DNA. DNA contains the genes that make up who and what you are. These genes are the instruction manuals for a cell. Cancer comes from mistakes made in copying the DNA. (See the glossary for a scientific definition of DNA.)

Mutations Are the Drivers of Cancer

Mutations are changes to DNA, the brain of the cell. These changes underlie the process a normal cell undergoes when it becomes a cancer cell. Mutations in DNA lead to abnormal instructions being sent to the rest of the cell.

Some risk factors for mutations include age, smoking, alcohol, weight gain, certain infections (such as HPV and hepatitis B or C), environmental exposures, genetics, and lots of other factors we don’t yet understand. Many of these risk factors mess with our DNA in inappropriate ways that lead to cancer.

Environmental causes of cancer include exposure to certain chemicals and other materials (these are called carcinogens—see the glossary for a scientific definition). An example of an environmental factor that can cause cancer is asbestos. Exposure to asbestos puts people at risk for a type of lung cancer (mesothelioma).

Why Age Is a Risk for Cancer

Age is one of the biggest risk factors for cancer. We see cancer much more commonly in older people compared to young people.

Each time a cell divides, it has to make a copy of its DNA. Random errors occur in the DNA during the copying process. This is a normal part of existence. Most of the time, the cell corrects these errors. As a cell gets older, it makes errors more often and corrects them less often. Therefore, the chance of mutation (changes in DNA) increases over time!

Cancer Cells Grow like Weeds:
Unlimited Growth in the Garden

Errors in DNA can cause cells to behave selfishly. The faulty DNA tells the rest of the cancer cell to divide (split into two cells). In simple terms, one cell becomes two, two cells become four, four cells become eight, and so on. After ten divisions, that one cell has led to more than a thousand others! This is where we get the definition for cancer—a disease caused by the unchecked growth (or division) of cells in the body.

Like weeds in a garden, cancer cells take up valuable space and resources in organs. Cancer cells push on the normal cells, leading to abnormal function of the normal cells of that organ. This may lead to terrible consequences for patients.

Solid Tumors

In general, we split cancers into two types: liquid tumors and solid tumors. Knowing which type of tumor we’re dealing with gives us information on how a cancer may behave.

Solid tumors are cancers that originate in any solid organ or structure in the body. This could be the lung, the liver, or even the heart! Solid tumors can (and unfortunately do) spread to other parts of the body. The movement of cancer cells from one part of the body to another is called metastasis.

With a solid tumor, cancer cells originate from a mass or lesion (compared to liquid tumors, which come from the bone marrow; see the next section). This mass may be cut out with surgery or treated with radiation, chemotherapy, or some combination of the two.

Liquid Tumors

Liquid tumors are cancers that make their home in the bloodstream. These cancers include the many types of leukemia.

The cells in your blood (liquid) come from the bone marrow. The bone marrow makes up the inside of large bones, such as the pelvis or spine.

The normal cells made in the bone marrow are vital for normal function of the human body. These cells include red blood cells, which carry oxygen throughout the body; white blood cells, which are the immune cells that fight infections; and platelets, which are cells that allow the blood to clot.

Mutations in the cells of the bone marrow often lead to these liquid tumors. Because blood moves throughout your body, these cells may travel anywhere blood goes. This makes surgery, which focuses on only one part of the body, less likely to be part of the treatment.

These definitions of solid and liquid tumors are useful, but they aren’t perfect. For example, a cancer called lymphoma falls in between: it forms masses in the lymph nodes but is often thought of as a liquid tumor. We’ll talk about lymph nodes later on.

How Does Cancer Decide Where to Go?

Cancer spreads anywhere it needs to go to find an environment with resources to help it grow. It’s clear that different types of cancers tend to spread to certain parts of the body. For example, prostate cancer tends to spread to bone. Breast cancer tends to spread to the armpit region (lymph nodes are hidden there). Lung cancers often spread in the chest or to the brain.

These patterns of spread can be very specific. Just like there are certain soils that are better for some plants, there are better places in the body for certain cancers to grow. This idea is called the seed and soil hypothesis. These specific patterns happen because cancer cells communicate with (send signals to) other organs.

Where Has All the Progress Gone?

We’ve been waging the “war on cancer” since 1971 (it started with the Nixon administration). Patients often express frustration at the lack of progress in treating cancer. Where has all that money in cancer research gone? Why aren’t we doing better?

First, let me say that doctors are frustrated too. We’ve definitely made strides in treating cancer, but we have a long way to go.

Treating cancer is hard for many reasons. One is that when cancer spreads to different parts of the body, it can keep on changing. As cancer cells grow, new changes in the DNA may allow them to grow faster.

Cancer is like a weed. A weed will use any advantage to go after sunlight, water, and nutrients in the soil. In the same way, a cancer cell will use any competitive edge for resources in the body. Weeds do this by growing taller and faster than many other plants to gain more access to sunlight. The deep roots of weeds are difficult to remove, allowing the weed to regrow if it breaks. Cancer cells, like weeds, will try to steal the nutrients from normal organs and cells need to survive. The changes cancer cells go through also allow them to be more resistant to chemotherapy, radiation, or immunotherapy.

Another problem is that cancer cells are so much like normal cells; they’re like closely related cousins. Most of the DNA between a normal cell and a cancer cell is the same. It can be hard to tell the cells apart.

In a garden, you want to make sure any method you use to get rid of weeds—pulling them out, spraying weed killer, etc.—doesn’t remove the local plants too. But at its core, a weed is still a plant. To get rid of weeds, you need to figure out what makes weeds different from other plants. In the same way, the only way to treat cancer is to find some differences between normal cells and cancer cells. Finding and exploiting these differences for treatments is hard.

Cancer’s Achilles’ Heel

Cancer cells have a deep desire to grow and divide. It drives everything they do.

This constant drive to divide is something we use to our benefit in trying to eradicate it. It’s the difference between cancer cells and normal cells. Taking advantage of the cancer’s basic instincts to divide is why chemotherapy and radiation are effective treatments. Some mutations (changes) in the DNA that allow cancer cells to grow also give us unique targets to hit with our expanding list of drugs. We’ll explore this idea in later chapters on types of cancer treatments.

Getting Back to the Basics

To summarize, cancer is common. It’s the unchecked division of cells in the body caused by mutations. These mutations happen because of age, exposures (known and unknown), and lifestyle choices.

Treating cancer is hard because it’s different in different people. It can move to a new part of the body and keep changing. At its core, cancer is just a shadow of us. The cancer cell has a lot in common with a normal cell. But for treatment, we use the cancer’s desire to divide against it.

In the next chapter, we’ll go back to basic biology to further discuss how cancer grows in relation to normal cells. This will give you some context on how cancer therapies work and what their side effects are.