By The Chronicle Staff

Cancer touches the lives of millions of Americans every year, either through a personal diagnosis or that of a family member, friend or neighbor.

According to the American Cancer Society, about one in three people in the United States will develop cancer during their lifetime, making it one of the nation’s most common health challenges.

While the word “cancer” often is used as though it describes a single disease, it actually refers to more than 100 different diseases that share one key characteristic: the uncontrolled growth of abnormal cells.

Understanding how cancer develops, the different forms it can take and how it spreads can help people make informed decisions about prevention, screening and treatment.

Healthy cells in the body grow, divide and eventually die in a carefully regulated process. Cancer begins when something disrupts that process.

Changes, or mutations, occur in a cell’s genes — the DNA that directs how cells function. Instead of dying when damaged, these abnormal cells continue growing and dividing, creating more abnormal cells.

Some gene mutations are inherited from a parent, while others develop during a person’s lifetime.

Lifestyle choices, exposure to certain chemicals or radiation, infections and aging can all contribute to genetic changes that increase cancer risk. In many cases, however, doctors cannot identify a single cause.

One hallmark of cancer is its ability to grow unchecked. As abnormal cells multiply, they crowd out healthy cells that perform the body’s normal functions. Many cancers also form tumors, or masses of abnormal tissue, although not every tumor is cancerous.

Doctors distinguish between malignant tumors, which are cancerous and can invade nearby tissue, and benign tumors, which are not cancer and generally do not spread.

Other abnormal growths, such as precancerous cells, may have the potential to become cancer over time but are not yet cancer.

Likewise, cysts — sacs filled with fluid, air or other material — are usually not cancerous and typically do not contain abnormal cells.

Cancer also is classified based on where it begins. A cancer that starts in the breast, for

example, remains breast cancer even if it spreads to the lungs. In that case, it is known as metastatic breast cancer rather than lung cancer because it originated in breast tissue.

Some cancers are named for the type of cell in which they begin. Carcinomas, the most common type, develop in the skin or the lining of internal organs. Sarcomas arise in bones, muscles or connective tissues. Other cancers originate in the blood or immune system.

Broadly speaking, cancers fall into two major categories.

Blood cancers, also called hematologic cancers, begin in blood-forming tissues or blood cells. Common examples include leukemia, lymphoma and multiple myeloma.

Solid tumors develop in organs or body tissues. Among the most common are breast, prostate, lung and colorectal cancers.

Although all cancers involve abnormal cell growth, they can behave very differently. Some grow slowly and remain confined to one area for years. Others spread rapidly throughout the body.

Certain cancers depend on hormones for growth, while others do not. These differences help determine the most effective treatment.

Because every cancer is unique, identifying the exact type — and often its subtype — is an important first step before treatment begins.

Modern laboratory testing allows doctors to examine cancer cells under a microscope and analyze their genetic characteristics. This information helps physicians select therapies that are most likely to be effective.

Treatment options vary depending on the cancer. Surgery may be used to remove a tumor before it spreads. Radiation therapy uses high-energy beams to destroy cancer cells in a specific area. Chemotherapy employs drugs that travel throughout the body to kill rapidly dividing cells.

Newer treatments, including targeted therapies and immunotherapy, attack specific features of cancer cells or help the body’s immune system recognize and destroy them.

Many patients receive a combination of treatments to achieve the best possible outcome.

Another critical factor in planning treatment is determining the cancer’s stage.

Staging measures the size of a cancer and whether it has spread beyond its original location. Doctors use imaging tests, laboratory studies and sometimes surgery to determine the stage.

In general, Stage 1 and Stage 2 cancers are smaller and have spread little or not at all, although some may involve nearby lymph nodes.

Stage 3 and Stage 4 cancers typically indicate more extensive spread to surrounding tissues, additional lymph nodes or distant organs.

Knowing the stage helps doctors estimate a patient’s prognosis and recommend the most appropriate treatment plan.

Perhaps the most dangerous characteristic of cancer is its ability to spread, a process known as metastasis. Cancer cells can break away from the original tumor and travel through the bloodstream or the lymphatic system to other parts of the body.

While many of these cells die before establishing new tumors, some survive and begin growing in a new location.

Metastatic cancer is generally more difficult to treat because it involves multiple areas of the body rather than a single site.

Although cancer remains a leading cause of death, advances in screening, diagnosis and treatment have improved survival rates for many types of the disease. Health experts continue to emphasize the importance of healthy lifestyle choices, recommended cancer screenings and prompt medical evaluation of persistent symptoms.

Understanding what cancer is — and what it is not — can help reduce fear, encourage early detection and empower patients and families to seek timely care when it matters most.■