Radiation pneumonitis is inflammation of the alveoli (tiny air sacs) within the lungs. The alveoli are involved in the passing of oxygen to the bloodstream and carbon dioxide from the blood to the outside world. In radiation pneumonitis the alveolar sacs become filled with fluid which interferes with the free passage of oxygen and carbon dioxide, this leads to breathlessness.

Symptoms can occur within the first few months following radiotherapy and include a persistent dry cough, breathlessness on effort, weakness, tiredness and sometimes fever.

Radiation pneumonitis is very uncommon and only rarely occurs after treatment for breast cancer. This is because the radiotherapy is carefully planned so that only a small amount of the lung is treated. It is possible that those women who do develop the condition have lung tissue that is particularly sensitive to the effects of radiation. The reasons for this increased sensitivity in some people are unknown.

The treatment of radiation pneumonitis is to relieve symptoms and prevent scarring to the lung. Steroids are usually given to reduce inflammation and ease breathlessness. Antibiotics may also be used to prevent, or treat, infection.

Symptoms usually improve with treatment and the condition resolves over two to three months.