What Causes Cancer?

The nucleus of every cell in the human body contains 23 pairs of chromosomes that are made of DNA molecules. Long sections of DNA molecules carry units of information called genes. These genes control all the processes in a cell by producing different kinds of proteins. When a cell divides, the DNA molecules duplicate themselves, and each new cell receives identical copies of the original genes so that it can continue to function normally.

But sometimes things go wrong. In 1914 German biologist Theodor Boveri proposed the idea that cancer was the result of abnormal chromosomes. The idea that cancer was somehow a genetic disease slowly gathered evidence through the twentieth century. Today we believe that changes in genes, called mutations, can convert normal cells into cancer cells. Then, when the cancer cells divide, the mutations in the genes are copied, so more cancer cells are produced. Mutations in perhaps 10 genes out of the 24,000 genes in the human body can cause a cell to become cancerous. These mutations can occur in two kinds of genes: oncogenes and tumor suppressor genes.

Oncogenes are mutations of normal genes that regulate cell growth and cell division, and can cause cancer cells to divide without stopping. In 1976 Michael Bishop and Harold Varmus of the University of California at San Francisco first isolated an oncogene, named src, from a type of chicken cancer. The normal src gene produces a protein that passes on signals that control cell growth. When this normal gene mutates (changes) to form the src oncogene, it produces a different protein that sends continual signals for the cell to grow. This protein is found in certain kinds of lung, colon, and breast cancers. More than 100 oncogenes have now been discovered and linked to specific forms of cancer.

Tumor suppressor genes are normal genes that produce proteins to control cell division and prevent cells with damaged DNA from dividing. If these genes are working properly, cells with mutations that could cause them to become cancer cells are prevented from dividing and forming tumors. In 1986 cancer researchers Thaddeus Dryja, Stephen Friend, and Robert Weinberg found the first tumor suppressor gene, RB. Mutations of the RB gene cause retinoblastoma, a rare cancer of the eye that occurs in infants. In 1989, researchers found the p53 tumor suppressor gene, which produces a protein that prevents damaged DNA from being copied during cell division and orders the cell to commit suicide by apoptosis. If the p53 gene mutates and can’t function, cancer cells will divide continually, even when their DNA is damaged, and produce a tumor. Mutations in the p53 gene are found in more than half of all cancers.

Can X-rays Cause Cancer?

X-rays are common in today’s world and are used for everything from screening baggage at airports to making images of organs and bones inside the body. Today we know that X-rays are electromagnetic radiation just like visible light, but the eye cannot see them because their wavelength is too short. They can, however, make an image on film and on special screens.

There is no doubt that X-rays and other forms of radiation can cause cancer. Yet people routinely have diagnostic X-rays at visits to their dentist and doctor, and radiation therapy is prescribed for about half of all cancer patients. Is there a contradiction here?

We are all exposed to radiation from natural sources. This background radiation includes electromagnetic radiation and high-energy particles from the sun, from radioactive elements such as radon, and from cosmic rays coming from space. The background radiation that a person receives each year is greater than what a person receives from routine dental and medical X-rays. A routine X-ray is equivalent to a few days of background radiation. Nevertheless, low doses of radiation from any source do increase the risk of cancer very slightly; however, medical scientists believe that the benefits of routine X-rays outweigh the risk of cancer.

Radiation therapy involves much higher doses of radiation than routine X-rays, so is the risk of cancer greater? The answer is yes. Leukemia, breast cancer, lung cancer, and other cancers may occur 5, 10, or 20 years after radiation therapy has cured the original cancer. Although the risk of developing a secondary cancer from radiation therapy is difficult to measure, it appears to be low. Some cancer patients worry about radiation therapy giving them cancer. However, the risk of dying from cancer that is not treated is much greater than the risk of developing cancer from radiation therapy.

Radiation therapy is effective in treating cancer because it damages the DNA of cancer cells to the point where it can no longer reproduce itself. Once bombarded by X-rays, gamma rays, or other forms of radiation, cancer cells stop dividing and die off. Radiation therapy can be done externally, with X-rays or a beam of electrons directed into the body, or internally, with radioactive isotopes inserted into or near the tumor. Certain kinds of cancer of the larynx, lung, cervix, prostate, thyroid, and brain are usually treated with radiation therapy.