Cancer

Cancer is a large group of diseases defined by uncontrolled cell growth.

This growth often leads to the formation of tumors. A tumor is a mass of cells that has undergone unregulated growth. Some tumors are benign, meaning they do not spread to other parts of the body. Other tumors are malignant, which means they have the potential to invade nearby tissues. Malignant tumors can also spread to distant locations through a process called metastasis.

This process is why cancer is such a complex medical challenge.

To become malignant, cells must undergo several specific changes known as hallmarks. These hallmarks allow cancer cells to bypass the body's normal rules. First, cells grow and divide even when they do not receive the proper signals. They also continue to divide even when they receive signals to stop. Malignant cells also avoid programmed cell death, which is a natural process for removing old cells. They can divide a limitless number of times. Furthermore, they promote angiogenesis, which is the construction of new blood vessels to supply the tumor. Finally, they gain the ability to invade local tissue and form metastases.

Metastasis follows a specific sequence of steps to move through the body. It begins with local invasion into the surrounding area. Next, cells undergo intravasation, where they enter the blood or the lymphatic system. The cells then circulate through the body to reach new locations. Once they reach a new site, they perform extravasation to exit the blood or lymph vessels. After entering the new tissue, the cells begin to proliferate, or multiply. They may also trigger further angiogenesis to support their new home. While cancers can spread anywhere, they often move to the lungs, liver, brain, or bones.

Humans have studied these growths for thousands of years. The term originates from the ancient Greek word *karkinos*, which means crab.

Ancient physicians like Hippocrates and Galen observed tumors with swollen veins. They thought these shapes resembled the legs of a crab. The word entered the English language in a modern medical sense around the year 1600. Today, scientists recognize that there are over 100 different types of cancers affecting humans. This variety makes finding a single cure very difficult.

Many factors contribute to the development of cancer. About 90% to 95% of cases are caused by genetic mutations from environmental and lifestyle factors. These factors can change the genes of a cell. Common environmental contributors include tobacco use, which accounts for about 25% to 30% of cancer deaths. Diet and obesity contribute to 30% to 35% of deaths. Infections from viruses, bacteria, and parasites cause approximately 16% to 18% of cancers worldwide. For example, hepatitis B and HPV are infectious agents linked to cancer. Only about 5% to 10% of cancers are due to inherited genetic defects.

Cancer affects millions of people and has a massive global impact.

In 2015, roughly 90.5 million people worldwide had cancer. By 2019, annual cases grew by 23.6 million people. There were 10 million deaths worldwide in 2019, a 21% increase over the previous decade. The economic cost of cancer is estimated at US$1.16 trillion per year. However, survival rates can be high with early detection. In the developed world, the five-year survival rate for children under 15 is about 80%. In the United States, the average five-year survival rate for all ages is 66%.

Different types of cancer are more common in specific groups. In males, the most frequent types are lung, prostate, colorectal, and stomach cancers. In females, the most common are breast, colorectal, lung, and cervical cancers. In children, acute lymphoblastic leukemia and brain tumors are the most common. However, in Africa, non-Hodgkin lymphoma occurs more often.

If non-melanoma skin cancers were included in annual totals, they would account for about 40% of all new cases. Risk also increases significantly as people age.

Medical science offers several ways to manage and treat these diseases. Doctors may use surgery to remove tumors or radiation therapy to target cells. Chemotherapy uses drugs to kill cancer cells, though some chemotherapy drugs are themselves carcinogens. Targeted therapy focuses on specific mechanisms within the cancer. A newer field called immunotherapy harnesses the patient's own immune system to fight the disease. For advanced cases, palliative care helps manage pain and symptoms.

Early screening is also vital for detecting cervical and colorectal cancers early.