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Radiation Therapy for Cancer: Radiation therapy for cancer can be either internal or external. In internal therapy, small pellets of radioactive material are implanted directly into the cancer. In external therapy, a beam of radiation is transmitted through the person's body into the cancer.

External radiation therapy for cancer produces a number of symptoms, depending on the amount of radiation and the area of the body treated. Nausea, vomiting, and loss of appetite may occur during or shortly after irradiation of the brain or abdomen. Large amounts of radiation to a limited area of the body often damage the skin over that area. Skin changes include hair loss, redness, peeling, sores, and, eventually, thinning of the skin and dilated blood vessels just beneath the skin's surface (spider veins). These changes increase the likelihood of skin cancer years later. Radiation to the mouth and jaw can cause permanent dry mouth, resulting in an increased number of dental caries and damage to the jawbone. Pus-filled pockets of infection (abscesses) may then develop. Radiation to the lungs can cause lung inflammation (radiation pneumonitis), and large amounts may cause severe scarring (fibrosis) of lung tissue, which can be fatal. The heart and its protective sac (pericardium) can become inflamed after extensive radiation to the breastbone and chest. High accumulated doses of radiation to the spinal cord can cause catastrophic damage, leading to paralysis. Extensive radiation to the abdomen (for lymph node, testicular, or ovarian cancer) can lead to chronic ulcers, scarring, and narrowing or perforation of the intestines.

Prolonged or repeated exposure to low doses of radiation from radioactive implants or because of job-related duties (as occurs with some health care professionals) may cause cessation of menstrual periods (amenorrhea) in women, decreased fertility or sterility, and decreased sex drive (libido).

Occasionally, severe injuries develop long after the completion of radiation therapy. Kidney function may decline 6 months to a year after a person has received extremely large amounts of radiation, resulting in anemia and high blood pressure. High accumulated doses of radiation to muscles may cause a painful condition that includes muscle wasting (atrophy) and calcium deposits in the irradiated muscle. Very rarely, these changes result in a cancerous (malignant) muscle tumor. Radiation-induced cancers typically occur 10 or more years after exposure.

from Radiation Injury (http://www.merck.com/mmhe/sec24/ch292/ch292a.html)