Neutron Therapy: A Rare and Special Kind of Radiation
Only three facilities in the United States offer neutron therapy, an especially powerful kind of radiation therapy shown to be effective against salivary gland tumors and some other forms of cancer. The Seattle Cancer Care Alliance (SCCA) is proud to be one of these. SCCA offers the unique Clinical Neutron Therapy System (CNTS) at the University of Washington Medical Center, one of our parent organizations.
Each year our doctors use neutron therapy to treat about 100 people with cancer. About 80 percent of these patients have tumors of their major or minor salivary glands. They come from all around the country and the world.
“This is the best nonsurgical treatment available for salivary gland tumors,” says George Laramore, MD, PhD, radiation oncology specialist. Laramore has treated patients with neutrons from all corners of the United States and as far away as Israel, Taiwan, Australia, and countries throughout Europe and South America.
“Because this is a rare tumor, there are not going to be very many neutron facilities in the country. We are very lucky that we have one here in the Northwest,” says Laramore. “We probably see more salivary gland patients than anywhere else in the world.”
Hope for Large or Inoperable Tumors
The University of Washington received a contract from the National Cancer Institute in the late 1970s to design and build a clinical neutron therapy facility and then participated in many research studies, called clinical trials, to determine when and how to use neutron therapy effectively. Doctors at the University started treating patients using neutrons at the CNTS in 1984.
The equipment for neutron therapy is about 10 times as expensive as the equipment for regular radiation therapy, and a neutron therapy facility has specific physical requirements. So it’s unlikely more facilities of this kind will be built in the U.S. “A neutron radiotherapy facility would probably cost $20 million to $25 million dollars today,” says Laramore, prohibitively expensive for most institutions, especially given the small number patients for whom this important therapy is useful.
When salivary gland tumors are detected early, the standard treatment is surgical resection, or removal. But when tumors are more advanced, there’s a risk that surgery will mean sacrificing the facial nerve, leaving patients with facial paralysis. Our surgeons use surgical techniques designed to spare the facial nerve, explains Laramore. But this may mean leaving some of the tumor behind, he says. At this point, neutron therapy is added to their treatment plan.
“We use neutron therapy when there are high-risk features postoperatively or when surgery is not possible,” says Laramore. He estimates that each year only about 500 patients in the country are diagnosed with salivary gland tumors considered large or inoperable.
Neutron therapy is especially good at controlling salivary gland cancer at the tumor site and in the same region. (Some patients still develop distant metastases, areas where the cancer spreads.) Neutron therapy also works best on tumors below 4 centimeters in diameter, controlling about 80 percent of tumors this size. But it is used with success on many larger tumors as well.
One of the early studies on neutron therapy for salivary gland cancer, started in the 1980s, was halted early because patients getting neutron therapy were doing so much better than others in the study. Researchers did not want to deprive the rest of the study subjects of the benefits of the neutron option.
Neutron therapy can be used to treat cancer in areas other than the salivary glands as well. For instance, at SCCA, we sometimes use neutron therapy to treat cancer of the bones, joints, and soft tissues (sarcomas); certain radio-resistant tumors, such as melanomas, renal cell cancers and thyroid cancers; and other cancers—especially when surgery is not an option.
These are generally situations where the size and location of the tumor or the patient’s general medical condition make it too difficult or risky to operate.
How Is Neutron Therapy Different?
Regular radiation therapy for cancer uses beams of electrons or photons (also called x-rays) to bombard cancer cells. The collisions release free radicals—unusually active atoms that have one or more unpaired electrons. Free radicals damage the DNA in the cancer cells, ideally killing them. Each dose kills only some of the targeted cells, depending on where the cells are in their life cycle. So doctors give multiple doses over time to reach more of the cells during the periods when they are most vulnerable.
Neutron therapy uses beams of neutrons to attack cancer cells. There are two advantages to using neutrons instead of electrons or photons. The first is that neutron beams are much more powerful. They deposit about 20 to 100 times as much energy into the target tissue as regular radiation therapy does.
The second is that neutron beams have a higher probability to damage both strands of a cell’s DNA, whereas regular radiation in general damages only one strand. This makes it harder for cells to repair neutron beam damage and harder for them to survive the treatment. So neutron therapy is a good choice in some cases when tumors are resistant to regular radiation.
Because neutron beams are so damaging, the risk of side effects on healthy tissue near the cancer site is greater. For this reason and because neutron beams tend to diffuse more, neutron therapy equipment includes many mechanisms designed to precisely focus and direct the beam and to block exposure to any surrounding tissue.
What Happens During Treatment
Before a patient receives neutron therapy, his or her doctor uses imaging techniques such as computed tomography (CT) scan, magnetic resonance imaging (MRI) and positron emission tomography (PET) scan to establish the exact location of the tumor and to decide exactly which spots to target with the neutron beam. Then the patient visits the neutron therapy facility for a treatment planning session. During this visit, a radiation oncology physician simulates the treatment, determining the best positions for the patient and the equipment and the appropriate dose of neutrons. This process is computerized.
“Our goal is to optimize the neutron beam to maximize damage to the tumor and minimize damage to any other tissue,” says Laramore. It may take up to a week to develop an optimal treatment plan and actually begin treatment. For treatment, the patient lays on a padded table, also referred to as a couch. A pillar supports the table from underneath. The radiation therapist can move the table up and down, sideways and lengthwise, and can swing and rotate it to get the patient in exactly the right position for the neutron beam. Most patients receive beams from several different angles, usually two to four.
The neutron beam comes out of a large arm called a gantry. The gantry can rotate 360 degrees to deliver the beam from above or below the patient or from the side. To accommodate the large gantry arm, the floor can open as the gantry turns below the patient.
At the end of the gantry near the patient is a section called the treatment head, the part from which the beam emerges. Attached to the head is a leaf collimator. The “leaves” are narrow rectangular pieces of steel. There are 40 leaves inside the collimator. A computer controls the position of each leaf according to the radiation treatment plan. This creates an opening that’s the right size and shape and in the right location for the patient’s tumor. This opening is called the field. The radiation oncologist checks each field.
When the neutron beam reaches the collimator, the steel leaves block areas of the beam that would otherwise hit healthy tissue; the opening formed by the leaves allows part of the beam to get through to the patient. The whole rotating assembly, including the gantry, treatment head, collimator, and gantry counter weights, weighs about 39 tons.
An actual treatment session takes about 30 to 60 minutes, depending on how complicated the treatment delivery scheme is. Most of this time is spent getting the patient and equipment into the right positions. The time when the neutron beam is on lasts only about 1 to 2 minutes from each treatment angle. Typically patients get 16 to 18 treatments over a period of 4 to 5 weeks.
Sometimes patients find it tiring or uncomfortable to lie still for their treatment; otherwise there’s no discomfort during the treatment when the beam is hitting the target area. The side effects from neutron therapy are similar to those from regular radiation therapy but may be more intense. They depend upon the area of the body that is being treated and may include:
- Skin irritation, redness, dryness or swelling in the treated area
- Hair loss in the treated area
- With radiation to the head: changes in taste, difficulty swallowing, mouth sores, dry mouth, or jaw tightness
During the treatment, the radiation therapist works from a nearby control room. An intercom allows the patient and therapist to communicate, and the therapist can see the patient and treatment room through video cameras.
Behind the Scenes
From the treatment room, the patient sees only the last few pieces of equipment involved in delivering their neutron beam. Behind the scenes—literally behind the wall that holds the gantry—are the many devices needed to generate the neutrons.
Here’s what happens before the beam emerges from the collimator: The whole process begins with hydrogen gas. A device called an ion source strips electrons off the hydrogen atoms creating protons. The ion source is located at the center of a particle accelerator called a cyclotron. The cyclotron then uses magnetic and electrical fields to spin the protons in a spiral, accelerating them. The CNTS cyclotron can speed up protons to about 30 percent of the speed of light. In the linear accelerators used in regular radiation therapy, particles travel in a straight line, not a spiral. To get protons to as high a speed as the cyclotron can, a linear accelerator would have to be several tens of meters long. This wouldn’t be practical in a treatment facility such as a hospital. The cyclotron is both more compact than a linear accelerator.
Once the protons reach the high energy level needed, they are guided out of the cyclotron into a beam line tube. The protons are guided down the beam line and through the gantry arm by multiple magnets which control the shape and direction of the proton beam. In the treatment head the protons strike a target made out of beryllium metal. The collision breaks apart some of the beryllium atoms, separating the neutrons (which are neutral—they have no electrical charge) from charged fragments. The charged fragments don’t travel far, but the neutrons are penetrating. They escape from the area of the reaction and travel through the collimator opening to the tumor site. Unwanted neutrons are absorbed in the collimator leaves and other shielding devices.