Radiation therapy of oral cancer
Second to surgery, radiation therapy is the most important mode of treatment for this cancer type.
Even if the patient is successfully operated, radiation therapy will be necessary in most cases postoperatively. This is because there is often microscopic disease remaining where the tumor was localized even when margins are considered free. There may also be microscopic disease in lymph nodes which drain from the original location of the tumor. The likelihood of this depends on the site of the primary tumor and stage/depth of infiltration.
In the head/neck region, there are many structures sensitive to radiation. These should be protected from radiation as much as possible. This is difficult in practice, and a small dose of radiation must be accepted for certain structures. It is therefore important to know the level of tolerance, which depends on fractionation (how the total dose is divided into small, daily fractions). This has long been based on empirical data, but over the last two decades has also been based on mathematical models.
Side effects must be expected. Curing the disease may be at high cost of quality of life, even if no maltreatment has taken place. It is therefore not correct to use the expression radiation damage in this context.
High-energy electromagnetic radiation is used, and sometimes, high-energy electrons. This is generated with a linear accelerator, a machine that accelerates electrons to the speed of light with subsequent braking. This will develop electromagnetic radiation - "bremsstrahlung".
Radiation therapy with today's technology occurs with millimeter precision. Two circumstances are therefore very important:
- Knowing the disease site and extent.
- The patient must lie in the same position for each treatment and not move during irradiation.
One of the characteristics of cancer is local spread, that is, when a tumor infiltrates its surroundings. This is not always as easy to identify with today's diagnostic methods. Optimal evaluation of the tumor often requires CT and MRI examinations and in some cases PET. Still, one must still operate with multiple safety margins to compensate for uncertainty of disease extension and daily variation in positioning.
In order for the patient to lie still during radiation therapy, customized masks are made for the head/neck. This is known as immobilization. Thereafter, a CT examination is performed while wearing the mask. On the CT images, the tumor and organs at risk are labeled. After this, irradiation is planned by a radiation oncologist and physicist. When this is completed, the patient is ready for simulation, which is modeling and drawing of the field of radiation. When this is done, treatment can start. The entire process takes about 1-2 weeks.