Radiation therapy is the use of high-energy particles or rays to damage cancer cells and prevent them from growing. Other names for radiation therapy include radiotherapy, x-ray therapy, and irradiation. Although some normal cells may be affected by radiation, the cells most susceptible to damage by radiation are fast-dividing cells such as those in the bone marrow and lining the digestive tract. Most normal cells appear to recover fully from the effects of the treatment.

In myeloma, patients receive radiation from outside the body. This external radiation (or external beam radiation), as it is called, involves the use of a machine that directs the high-energy rays at the body.

Patients may receive external radiation to various parts of their body. They may receive localized radiation that is directed at a particular area of bone or radiation that is directed at a larger part of their body. This type of radiation therapy is typically administered on an outpatient basis at a hospital or cancer treatment center. Patients may also receive radiation of their entire body, also called total body irradiation (TBI) in preparation for a stem cell transplant.

Radiation therapy may be used in several ways in myeloma.

• Local radiation therapy at higher doses (with chemotherapy in some cases) is used in the treatment of solitary tumors in bone or soft tissue (plasmacytomas)
• High-dose radiation to a larger part of the body may be used to reduce tumor burden or as salvage therapy
• Local low-dose radiation therapy is sometimes used as palliative treatment to relieve uncontrolled pain and is also used to help prevent or treat bone fractures or spinal-cord compression
• Total body irradiation is used in conjunction with high-dose chemotherapy prior to stem cell transplantation in order to help kill myeloma cells in the bone marrow

In general, radiation therapy can make patients feel tired and lose their appetite, and can also make the skin above the treated area more sensitive and irritated. Other side effects of radiation depend on the site treated. For example, radiation to the pelvic area may also cause suppression of the bone marrow and lead to reduced blood cell counts. This is because half of the body's bone marrow is found in the pelvic bones. Radiation to the pelvic area can also affect the lower digestive tract, causing symptoms such as diarrhea, spasms, and in rare cases, bleeding. It can also affect the reproductive organs.