Cancer is a disease in which cells grow and divide without control, forming large tumors that can spread to other parts of the body. This process is called metastasis. Cancer is caused by a combination of genetic changes and environmental factors. Most of the genetic changes that lead to cancer occur after birth, but some happen before birth. These genetic changes can be inherited or result from certain exposures, such as tobacco smoking and sun exposure. The genetic changes that contribute to cancer usually affect genes that control how cells grow, repair DNA, or respond to hormones. These genetic changes may also affect the signals that tell cells to turn genes on or off.
Most of the time, when a cell in a multicellular organism needs to proliferate, it gets the signal from its environment to do so. As the population of a cell grows, it has to balance its growth with limits on resources, crowding, and exposure to toxins. Cells that can no longer withstand these stresses are eliminated by natural selection.
When this happens to the cells that make up a tumor, the resulting population is characterized by an accumulation of mutations over time. This enables cancer cells to outcompete normal cells and proliferate more quickly. It also enables them to avoid apoptosis, the normal mechanism that causes cells to stop growing and die when they are no longer needed. The accumulating mutations in a cancer cell population also allow the cells to adapt to their local environment and to resist treatment.
A number of treatments are available to fight cancer. These include chemotherapy, radiation therapy, and surgery. The type of treatment your doctor recommends will depend on the type and stage of cancer, as well as your general health and other factors.
The earliest sense of cancer as uncontrolled cell proliferation was consistent with pre-evolution perspectives that emphasized the role of cells and their changes. However, the more recent sense of cancer as a predictable evolutionary phenomenon of heritable variation interacting with the forces of natural selection is more compelling.
This new understanding of cancer is reflected in how it is defined. Current definitions of cancer focus on the characteristics of a tumor, such as size, location in the body, and whether it has spread beyond a single organ. However, most of these definitions fail to describe how the underlying genetic changes cause the formation of a tumor or its progression.
The outlook for people with cancer has improved dramatically over the past several decades. Advances in screening, improved treatments, and a drop in smoking have contributed to a year-on-year decline in cancer diagnosis and death rates. Despite these gains, people who have cancer are still at risk of developing more aggressive or resistant forms of the disease. Therefore, it is essential to continue researching new ways to prevent and treat cancer.