In proton therapy, a high energy beam of protons, instead of high energy X-rays, is used to deliver a dose of radiation therapy to cancer patients.
What is proton therapy
A proton therapy treatment gantry, such as this one at Roberts Proton Therapy Center, delivers precise proton beams to difficult-to-reach tumors. Photo credit: Penn Medicine's Roberts Proton Therapy Center and Ed Cunicelli.
Proton therapy is similar to radiation therapy, but it offers a more targeted approach. This means that the risk of damaging tissues around the tumor is lower than with standard radiation.
The treatment is considered suitable for some specific cancers that involve tumors near important parts of the body, such as the eye, brain, and spinal cord.
With traditional radiation therapy, such tumors cannot be targeted because of the risk of damage to vital surrounding tissue (nerves).
Proton therapy may also be used to treat cancers that affect:
- the central nervous system (CNS)
- the eyes
- the head and neck
- the lungs, liver, or prostate gland
- the soft tissues of the spine and pelvis, known as sarcoma
- non-cancerous brain tumors
The MD Anderson Center at the University of Texas describes proton therapy as a 196-ton, cancer-killing machine with sub-millimeter precision that can target a patient's tumor "while sparing nearby healthy tissues and minimizing side effects. In its most simple terms, that's proton therapy."
Although proton therapy is said to be a better-targeted form of treatment, there is some disagreement on whether it provides an overall advantage compared to other, cheaper therapies.
Proton therapy versus standard radiation therapy
According to the American Society of Clinical Oncology (ASCO), proton therapy may deliver up to 60 percent less radiation to healthy tissue around the target site, while delivering a higher dose to the tumor itself.
In proton therapy, the doctor can decide exactly when and where the proton releases most of its energy. This point is called the "Bragg peak." This means it is possible to inflict maximum damage to cancer cells and minimum harm to nearby tissue.
In standard radiation therapy, the X-ray beams deposit energy along their path before hitting their target, for example, on the body's surface and beyond. The X-ray beam continues beyond the tumor, releasing energy and harming tissue. This is called the "exit dose."
In other words, treatment hits the targeted cancer cells, but it also affects those along the X-ray beam before and past the tumor. This can lead to health problems after treatment.
With proton therapy, the doctor can use a higher dose of radiation than is possible in standard radiation therapy, but they can also protect surrounding tissue and vital organs.
In standard radiation dose, a lower-than-desired dose must be used to minimize the damage to healthy cells.
As you can see from the above diagram, the proton beam (blue) does less damage before hitting the tumor, and virtually none at all beyond it. However, the x-ray beam (red) spreads radiation before and beyond the tumor site into deep tissue at much higher levels.
Proton therapy adapts to tumor shape
Tumors come in all shapes, sizes, and locations, and they are unique to each individual. With patient-specific hardware, the radiologist can sculpt the proton beam, customizing it to strike within the borders of the tumor, whatever its shape might be.
The tumor can be hit with proton beams from different directions. This further ensures that damage to surrounding cells is kept to a minimum, reducing the risk of complications usually associated with radiation therapy.
The reasons for choosing proton therapy treatment can be divided into two broad categories.
Where higher dosages are needed: Proton therapy is used for tumors that need higher radiation doses, known as dose escalation.
Dose escalation has, in some cases, been shown to provide better outcomes for patients than conventional radiotherapy.
Cases where it is successful include:
- unresectable sarcomas
- uveal melanoma (ocular tumors, tumors in the eye)
- paraspinal tumors (chondrosarcoma and chordoma, alongside the spinal column)
To reduce unwanted side effects: Proton therapy can reduce the risk of undesirable side effects by limiting the damage to normal, healthy tissue, even if the dose is the same as in conventional therapy.
Examples of when this could be useful include prostate cancer and cancers that affect the prostate, the spine, the head, and the neck.
Proton therapy is especially useful for treating childhood cancers, because the cancer cells can be targeted without damaging other cells in a growing body. Children receiving traditional radiation treatment have a higher risk of stunted growth.
Proton therapy used alongside other therapies
In 2014, a team of oncologists reported in the journal Radiotherapy and Oncology that proton therapy "offered an additional gain" for patients with early-stage Hodgkin lymphoma after involved node radiotherapy.
Secondary cancer risk
Traditional radiation therapy is associated with a high risk of developing secondary cancers.
In 2013, a team of oncologists published results of a study that was carried out to determine whether proton therapy and traditional radiation therapy (conformal radiation therapy) might increase the risk of secondary cancer in normal organs among patients with neuroblastomas.
Neuroblastomas are cancers that develop from immature nerve cells located in different parts of the human body. They typically arise in and near the adrenal glands.
They found that radiation doses observed in normal, healthy organs were lower among patients receiving proton therapy compared to those receiving traditional radiation therapy.
The researchers concluded that while traditional radiation therapy increases the risk of secondary cancer in most organs, proton therapy appears to reduce the risk.
What to expect
The procedure begins with a simulation.
The medical team, usually a radiation oncologist and a radiation therapy nurse, will make and fit the person with a device that holds the body still while treatment is underway.
The type of device used depends on the location of the tumor. People with cancer that affects the head will have a special mask fitted. If the target area is the body, leg, or arm, a cradle device is made.
Proton treatment usually occurs up to 10 days after the simulation. The marks on the skin must not be washed off.
The cyclotron creates and accelerates the protons. Image courtesy of Varian Medical Systems, Inc. All rights reserved.
The individual is placed in a donut-shaped device known as a gantry, which rotates around the person and points the protons directly at the tumor.
A synchrotron, or cyclotron, creates and accelerates the protons. The protons are then removed from the synchrotron and magnets direct them to the cancer cells.
While the photon therapy is underway, the radiation therapist is in another room. However, they will see and talk to the patient using a two-way intercom and closed-circuit television.
The treatment does not usually take more than 2 minutes, and there should be no discomfort.
After this, the radiation therapist comes back into the room and removes the device that helped keep the person still during treatment.
Proton therapy is an outpatient procedure. Depending on the type of cancer, treatments may be spread over several weeks.
Side effects may occur, but they will be mild compared with the adverse effects of X-ray radiation. There may be redness around the area being treated and some (temporary) hair loss.