Radiation Therapy can be given externally in the form of X-ray beams, gamma rays or beams of subatomic particles. Treatment with external radiation is usually painless and takes 5 to 15 minutes per session. The number of treatments varies for each person and completion of therapy can take several weeks of closely scheduled sessions. Radiation can also be delivered internally. Radioactive substances are either placed inside a body cavity or implanted inside the tumor itself. During radiation therapy, normal cells located near the cancer cells may be damaged, but these normal cells are usually able to survive and recover.