Radiation causes side effects by damaging the cells in the treatment site. Some patients experience very little or no side effects while other patients may have more severe side effects.

Each patient's side effects depend on many factors: the area of the body treated, the number of treatments, the size of the treatment field and the dose of radiation. Radiation side effects also vary according to the age and physical health of the patient. The patient’s pre-existing medical conditions may slow the recovery.

Two common side effects of radiation are fatigue and decreased appetite. These side effects most likely represent the body's response to healing cell damage. Feeling tired or fatigued from radiation usually starts about the second to third week of treatment and usually increases until the end of treatment. Fatigue continues for a few weeks after radiation and gradually resolves as the body heals itself. Your family and friends need to understand that fatigue is a side effect of treatment and that you will require more rest or sleep and may not be able to work or perform your daily activities.

In lung cancer, radiation is aimed at the chest. The esophagus (food pipe) is located centrally in the chest. A common side effect is esophagitis, or inflammation of the food pipe. This can cause mild to severe discomfort when swallowing. Medications can be given to ease the discomfort. You may also need to alter the foods you eat. In some cases, the discomfort becomes so severe that the patient must be admitted to the hospital for intravenous fluids and pain medication. This side effect is self-limited and will heal completely with time.