External radiation therapy involves the use of high beams, generated from a machine outside the human body, and directed towards the tumor in order to shrink or kill it.

External radiation therapy is used to treat various types of cancers and is usually administered on an outpatient basis. It is also widely used as a palliative treatment. It is particularly used for those cancers that are difficult to remove or have a high chance to return back such as mesothelioma. External radiation therapy may be administered prior to surgery in order to shrink the tumor. This is called neoadjuvant therapy. It can be administered during surgery where a high-dose of radiation is beamed directly at the tumor site after the tumor has been partly or totally removed. External radiation therapy can also be administered after surgery in order to diminish the chances of the tumor coming back. This is called adjuvant therapy.

Patients being treated by external radiation therapy are not radioactive and visitors and nurses can safely stay nearby without any harmful consequence. External radiation therapy for mesothelioma may last from three to five weeks, where radiation is usually given for about five days a week.