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, also called Proton beam therapy, is the most advanced radiation therapy available today – it destroys cancer cells but does not attack surrounding healthy tissue nearly as much as traditional radiation therapy does. Proton therapy, a kind of particle therapy, directs proton beams with great precision at cancer cells.
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.
This Medical News Today article provides an easy-to-understand and in-depth explanation on what proton therapy is, how it differs from standard radiation therapy and it’s advantage regarding tumor shape. We also look at which cancers proton therapy is being used for today, and its relative safety regarding secondary cancer risk. At the end of the article you can learn what to expect before and during a proton therapy session.
What is 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 much cheaper therapies.
The National Health Service1 in the UK says that proton therapy is better with some rare cancers where the tumors are located at the base of the skull or spine. With traditional radiation therapy, such tumors cannot be targeted because of the risk of damage to vital surrounding tissue (nerves).
The MD Anderson Center2 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.”
The difference between proton therapy and standard radiation therapy
Standard radiation therapy – the X-ray beams deposit energy along their path before hitting their target (e.g. on the body’s surface) and also beyond. The X-ray beam continues beyond the tumor, releasing energy and harming tissue – this is called the “exit dose”.
In other words, the targeted cancer cells get hit, but so too do those along the X-ray beam before and past the tumor. This can lead to health problems after treatment.
Proton therapy – the doctor can decide exactly when and where the proton releases most of its energy. This point is called the “Bragg peak”.
The health care professional can determine the exact location of the Bragg peak, thus inflicting maximum damage to cancer cells and minimum harm to nearby tissue.
Radiation dose – with standard radiation therapy, a lower-than-desired dose has to be used to minimize the damage to healthy cells.
With proton therapy, on the other hand, the doctor can use much higher radiation doses while at the same time protecting surrounding tissue and vital organs.
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 (pink) spreads radiation before and beyond the tumor site into deep tissue at much higher levels.
Proton therapy adapted to tumor shape
Tumors come in all shapes, sizes and locations, and they are unique for each patient. 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, further ensuring that damage to surrounding cells is kept to a minimum, thus reducing the risk of complications usually associated with radiation therapy.
Applications of proton therapy
Proton therapy treatments can be divided into two broad categories:
- Where higher dosages are needed – proton therapy is used for tumors that require the delivery of higher radiation doses, known as dose escalation.Dose escalation has, in some cases, been demonstrated to provide better outcomes for patients than conventional radiotherapy. Examples include unresectable sarcomas, uveal melanoma (ocular tumors, tumors in the eye), and paraspinal tumors (chondrosarcoma and chordoma, alongside the spinal column).
- To reduce unwanted side effects – here the focus is not on a better chance of a cure but to minimize the undesirable side effects by limiting the damage to normal, healthy tissue.The radiation dose is the same as in conventional therapy. Examples include pediatric neooplasms and prostate cancer.
The US National Library of Medicine6 says that proton therapy is most commonly used for treating tumors in the:
[a study published in JAMA questioned whether proton treatment for prostate cancer offered additional benefits]
- Head and 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 can be used alongside other therapy can be used alongside other therapies
A team of oncologists wrote in the journal Radiotherapy & Oncology4 that for patients with early stage Hodgkin lymphoma after involved node radiotherapy, “PT (proton therapy) offered an additional gain”.
Secondary cancer risk – traditional radiation therapy versus proton therapy
Traditional radiation therapy is associated with a high risk of developing secondary cancers.
A team of oncologists carried out a study 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.
In the journal Radiation Oncology they wrote that radiation doses observed in normal (healthy) organs were much lower among patients receiving proton therapy compared to traditional radiation therapy.
The researchers concluded that while traditional radiation therapy increases the risk of secondary cancer in most organs, proton therapy had the opposite effect, it reduced the risk5.
What happens during a proton therapy session?
The medical team, usually a radiation oncologist and a radiation therapy nurse, will make and fit you with a device that holds your body still while treatment is underway. The type of device used depends on the location of the tumor. Head cancer patients will have a special mask fitted. If the target area is the body, leg or arm, a cradle device is made.
An MRI (magnetic resonance imaging) and/or CT (computed tomography) scan will be taken to map out the area to be treated. The team will mark your the skin where the beam should be aimed at.
Proton treatment usually occurs up to ten days after the simulation. The marks on the skin must not be washed off.
You will be placed in a gantry (a donut-shaped device). It rotates around you and points the protons directly at the tumor.
A synchrotron (cyclotron) creates and accelerates the protons. The protons are then removed from the synchrotron and magnets direct them to the cancer cells.
The cyclotron creates and accelerates the proton. Image courtesy of Varian Medical Systems, Inc. All rights reserved.
While the photon therapy is underway the radiation therapist is in another room. However, he or she will see and talk to you by a two-way intercom and closed circuit TV.
The treatment does not usually take more than two minutes and there should be no discomfort.
When it is over, the radiation therapist comes back into the room and removes the device that helped keep you still.
Proton therapy is an outpatient procedure. Depending on the type of cancer, treatments may be spread over several weeks.
Side effects – there may be some side effects, however, they will be much milder compared to X-ray radiation. There may be redness around the area being treated and some (temporary) hair loss.