DEAR MAYO CLINIC: What does proton beam therapy do for cancer patients that standard radiation therapy doesn't do? How do doctors decide when to use proton beam therapy?
Proton beam therapy is a type of radiation therapy used to treat cancer. Unlike standard radiation therapy using X-rays, which travel all the way through a person's body, protons go to the tumor, release their energy and stop. That means proton beam therapy tends to be more effective, and causes fewer side effects, than standard radiation therapy.
Protons are subatomic particles that combine with neutrons to form the nucleus of an atom surrounded by orbiting electrons. Radiation is energy released from atoms as either electromagnetic waves -- such as X-rays or gamma rays -- or as tiny particles, such as electrons or protons. For more than 120 years, radiation has been used to destroy cancer cells.
Today's standard radiation therapy uses high energy X-rays that travel through the body. Proton beam therapy is different. This treatment directs protons into a tumor, where their energy is released. Radiation oncologists can control the depth of penetration of the protons and where they release their energy by adjusting the energy of the protons. The higher the energy, the deeper the protons go.
The amount of radiation energy released as the proton is entering a person's body is quite low. The majority of the energy released is in the last few millimeters of the protons' path, so the largest amount of radiation energy is released directly within the tumor.
For example, consider a person who has a tumor located near the back of one lung. The standard X-ray beam aimed at the tumor from the back will travel through the tumor and exit through the normal lung and heart in front of the tumor. With proton beam therapy, protons also are directed into the body from the back, but they stop in the tumor. That delivers radiation to the tumor, but none to the normal lung and heart in front of it.
Because much of the healthy organs surrounding the tumor do not receive radiation energy with proton beam therapy, side effects are less likely and less severe when compared to standard radiation therapy.
The radiation energy can also be more accurately and precisely controlled using proton beam therapy. That means the amount of radiation energy delivered to the tumor can often be safely increased, potentially increasing the treatment's effectiveness and possibly decreasing the number of treatments needed by giving a higher dose with each treatment. This makes the treatment more convenient for the patient and less costly.
Proton beam therapy can be used for many kinds of tumors. It's particularly useful in cancers located near critical organs that are sensitive to the effects of radiation. It's also well-suited to treating tumors located deep within the body, when concern about damage to healthy organs and tissue may require that the standard radiation treatment dose be decreased. In particular, proton beam therapy is usually considered a good option for brain, head and neck, esophageal, breast, liver and lung cancers in adults.
Proton beam therapy is ideal for children. Radiation therapy can cause cancers, heart disease and other chronic health problems decades later in children and young adults who are cured of their cancer by radiation therapy. Proton beam therapy lowers that risk of chronic health problems because the child's body is exposed to a lower dose of radiation when compared to standard radiation therapy.
Although it has a number of advantages, proton beam therapy is unlikely to completely replace standard radiation therapy. Standard radiation is typically a better choice for superficial cancers like skin cancer. It's also preferable for elderly patients who have other medical illnesses, as well as for treating symptoms associated with cancers that have spread. -- Robert Foote, M.D., Radiation Oncology, Mayo Clinic, Rochester.