Radiation therapy is the old standby for many cancer treatments, and mesothelioma is no different.
1. Why use radiation?
High dose radiation is used to directly kill cancerous cells in the body. The radiation destroys cancer cells at the molecular level, and keeps them from replicating. Radiation is usually combined with chemotherapy and surgery to form a total front against cancer.
2. Where does radiation come from?
In previous years, radiation from high energy X-Rays, photons, neutrons, cobalt or other radioactive sources were concentrated on the tumor, and this radiation would kill the cancer cells and prohibit them from growing and spreading. This radiation would come from a machines and radioisotopes, but technology has developed several promising new advances.
3. What does using radiation to treat mesothelioma accomplish?
Radiation is very useful against cancer of cells that divide quickly, like mesothelioma, because it kills cells that replicate fast. When patients undergo radiation therapy, they receive several successively higher doses of radiation, which shrink the tumor to a manageable size. Radiation relieves a great deal of pain, and victims of pleural mesothelioma suffer reduced instances of shortness of breath. Unfortunately, radiation cannot kill all of the cancerous cells without causing significant damage to the patient.
4. What kinds of mesothelioma radiation therapy are available?
There are two main types of radiation therapy: internal and external

• Internal radiation therapy (also known as brachytherapy) injects small amounts of radioactive material directly into the cancerous tissues, usually through tubes, wires, catheters, and needles.  This allows higher does of concentrated radiation over a shorter time frame on more entrenched tumors.
• External radiation therapy is the standard type of radiation therapy.  Over the course of a day, weeks, or months, radiation is delivered into a patient to stop the spread of cancer throughout the body.

5. What are the side effects of mesothelioma radiation treatment?
The problem with this therapy is that radiation is non selective and does not just kill bad cancer cells, but also many other healthy cells as well. Most healthy cells being to repair themselves after initial exposure, but this damage and repair are what cause the unpleasant side effects of radiation. Most of the side effects of the radiation are manageable with drugs or other pharmaceuticals, and the benefits usually far outweigh the risks. Some common side effects include:

• Redness near the irradiated area
• Infertility
• Nausea
• Vomiting
• Loss of appetite
• Fatigue
• Hair loss
• Dry mouth
• Diarrhea

6. What can affect mesothelioma radiation treatment?
Many different variables affect radiation treatment of a mesothelioma tumor. Size, stage, location, and type are all factors considered by doctors when pursing the best route and amount of radiation therapy. The treatment is often given from several angles to best target the site of the tumor. The radioactive material and the dead cancer cells are eliminated from the body through normal excretory processes, and do not usually harm the cells after the treatment. The only downside is that the size and location of the tumor may prevent radiation from properly affecting the target tumor, requiring several treatments.

Mesothelioma Radiation

In addition to chemotherapy and surgery, radiation therapy is one of the three most commonly used treatments for mesothelioma.
Radiation therapy makes use of a type of radiation called "ionizing radiation," which is used to kill existing cancer cells and control the growth of new cancer cells. Depending on the nature of the patient's condition and their type of asbestos cancer, radiation therapy may be used as a stand-alone treatment or may be used in conjunction with surgery or chemotherapy.

In addition, radiation may be used either as a curative treatment or as a palliative treatment. In the latter case, the purpose of radiation therapy is not to cure the patient, but simply to provide relief from pain and other symptoms of the cancer. This is often the case for mesothelioma patients that are prescribed radiation therapy.
There are three main types of radiation therapy: external beam radiotherapy, unsealed source radiation therapy, and brachytherapy. In most cases, only external beam radiotherapy and brachytherapy are used to treat mesothelioma. The type of radiation treatment a patient receives depends on several factors, including the type of mesothelioma, the stage of the disease, and whether other treatment options are also viable.

External Beam Radiotherapy

External beam radiotherapy is the most common type of radiation therapy for the treatment of cancer. During this procedure, the patient sits or lies down to expose the area that requires treatment, and an external source of radiation is directed at the treatment area.
Over the course of a series of treatments, mesothelioma tumors are bombarded with radiation to kill cancer cells and limit the growth and spread of these cells. However, in most cases involving mesothelioma, radiation therapy is not able to completely eradicate all tumor cells. As such, this type of therapy is usually given to provide the patient with some relief from pain and other symptoms associated with mesothelioma, such as shortness of breath and coughing.
Due to the nature of the treatment, it is common for healthy tissue near the treatment area to be affected by radiation. This damage causes side effects such as skin irritation, pain near the treatment site, digestive problems, and difficulty swallowing.
External beam radiotherapy is rarely effective as a stand-alone treatment for mesothelioma and is most often used in conjunction with chemotherapy or surgery.

Mesothelioma Brachytherapy

Brachytherapy, also known as sealed source radiation therapy, is a procedure that involves the implantation of tiny radioactive rods in or near tumors of mesothelioma and other types of cancer such as breast, cervical, and prostate cancer. This treatment method allows the delivery of a highly-concentrated and closely targeted dose of radiation to mesothelioma tumors that might be resistant to other types of radiation therapy.
This type of radiation therapy is beneficial because it causes little damage to healthy cells surrounding the affected area and is less invasive than traditional radiation.
Patients with mesothelioma may receive either temporary or permanent brachytherapy treatment. For temporary treatment, radioactive seeds are placed within tumors for a short time and are then removed. In the case of permanent treatment, the seeds are not removed, but are simply left within tumors and eventually cease the emission of radiation.

Unsealed Source Radiation Therapy

Treatment with unsealed source radiation involves the administration of soluble forms of radioactive substances. This type of treatment is administered either orally or via injection, but is only used to treat very specific types of cancer. For example, patients with thyroid cancer are often treated with a solution of radioactive iodine.
One major disadvantage of unsealed source radiation therapy is that people who receive this treatment are considered to be radioactive for the duration of the treatment and for several weeks after - this means they can actually pose a health risk to other people. Note that this is not a treatment that is used for patients with mesothelioma.
Sources:

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2. Galateau-Salle, Francoise. Pathology of Malignant Mesothelioma. Springer-Verlag London Limited: London. 2006.
3. Pass, I., Vogelzang, N., Carbone, M. Malignant Mesothelioma: Advances in Pathogenesis, Diagnosis, and Transitional Therapies. Springer: New York. 2005.
4. http://www.cancer.org/downloads/PUB/DOCS/SECTION28/89.pdf
5. http://www.americanbrachytherapy.org/aboutBrachytherapy/BrachyFAQFinal2.pdf