Radiation Carcinogenesis

3. Radiation Carcinogenesis

There is clear evidence that radiation can result in cancer in human beings. Although at present the number of tumors induced by artificial radiation constitutes only a very tiny fraction of all human cancer, the potential will increase because of increasing use of radioactive substances in industry and medicine.

Irradiation of cells by X rays, ultraviolet light, and other physical sources can be broadly categorized into two pathological results:

1. Direct mutagenic effects—an  immediate physical  or chemical event at the molecular level, within the fraction of a second.
2. Indirect mutagenic effects—an alteration of a normal balance within the organism and a subsequent neoplastic transformation in the cell through loss of normal resistance or control.

3.1 Direct Effect of Irradiation on Cells

Physical particles, such as X rays, gamma rays, fast electrons, alpha particles, and ultraviolet particles, carry high amounts of energy and when they strike a cell, a direct change occurs in the cell’s chemical organization and genetic structure.

The nucleus of the cell is far more responsive to radiations than its cytoplasm and in the nucleus itself the RNA is the macromolecule affected. It is therefore logical that the function of the cell most sensitive to radiation is the reproductive one since it is engineered by the nuclear DNA.

3.2 Indirect Induction of Cancer by Radiations

Dr. Henry Kaplan and his associates at Stanford University (from 1950 to 1970) have done the most to elucidate the mechanism of irradiation induction of cancer in mice. He has essentially proven that the induction of cancer by irradiation is an indirect, not a direct, mutagenic effect. Lymphatic leukemia of the thymus in mice can be induced only if the thymus is irradiated following irradiation of the mouse’s entire body.

When the upper part of the body alone was irradiated, the spleen and bone marrow cells acted as protective agents against the induction of cancer. When the bone marrow and the spleen were also irradiated, their protective capacity was annihilated and the mice developed thymic tumors.

3.3 Effects of Radiations

The indirect induction of cancer by radiations also has implications in man, particularly with respect to human leukemia.

The carcinogenic effects of irradiation have been shown for a variety of cancers in man. Sarcomas occur from ingestion of radioactive isotopes, such as those deposited in the long bones of radium dial painters; pulmonary carcinomas in underground mine workers result from exposure to alpha radiations in high concentration’, from radon in the air of underground mines. In addition it is possible that polonium, existing in low doses in tobacco, is a carcinogenic agent in cigarette smoke.

Patients treated by irradiation for ankylosing arthritis of the spine and the survivor; of the Hiroshima und Nagasaki atomic blasts show an increased or excessive incidence of leukemia. Breast cancer is more frequent in women undergoing multiple fluoroscopic examinations in the treatment of pulmonary tuberculosis. A single x-ray film of the abdomen of a pregnant woman produces a significant increase in the incidence of cancer, including leukemia, in the child.