Deadly
Accuracy
Proton beam therapy kills cancer cells
without the collateral damage of traditional radiation.
By Katy Human
Skin burns. second cancers. Sterility.
The possible side effects of conventional radiation therapy
are frightening. It’s a potent weapon against cancer, physicians
say, especially with today’s technical advances, and it’s
less damaging to healthy tissues than it was 10 years ago.
But an accumulating pile of studies are drawing attention
to an older radiation treatment that pounds cancer with far more
energy at the tumor’s exact location than conventional radiation
and saves normal tissues from extensive injury. It’s called
proton beam therapy, and medical experts call it the next generation
of radiation therapy.
Despite its modern precision, proton beam therapy has actually
been around since the 1950s, when physicists began to understand
the compelling differences between proton beams and intense X-ray
beams, called photons, used in conventional therapy. Those differences
let doctors control the depth of proton beams more precisely and
deliver more cancer-killing radiation to tumors—a particularly
appealing effect for discrete tumors of the eye and for treating
children, who are most vulnerable to the damaging side effects of
radiation. Proton beam therapy is also used to treat some prostate,
brain, lung, esophageal and head and neck cancers, among others.
“I don’t love military analogies, but it really
is like having a smart bomb. You can avoid collateral damage,” says
James Cox, MD, head of the radiation oncology division at M.D. Anderson
Cancer Center in Houston and medical director of its new Proton
Therapy Center.
Physics and Cancer
Protons are minute particles with a positive charge (see sidebar,
page 20), accelerated to great speed by equipment once familiar
only to research physicists. Because protons have mass, they work
on impact with the tumor and don’t travel all the way through
the body.
Conventional X-rays are a form of electromagnetic radiation
with short wavelengths. Without charge or mass, X-rays irradiate
cells continually as they pass through the body, delivering injury
both at the surface of the body where they enter, and in tissues
behind the cancer.
“Protons are different. Their dose is deposited with
a burst of energy where you want it—in the tumor,” Dr.
Cox says. “The location is determined by the energy of the
proton beam.”
Doctors can determine exactly where they want to deliver the
bulk of the cancer-busting power of protons (see illustration).
By accelerating protons to different speeds, they can target a tumor
just under the surface of the skin, or one deep inside a body cavity.
Some call this the “depth charge effect,” because
it’s akin to dropping a bomb into the water, and determining
the precise depth at which it detonates.
Although side effects are still possible, they are considerably
less intense than with conventional radiation and are far less likely
to be long-term. More common symptoms include skin irritation and
hair loss in the direct path of radiation, and fatigue if a large
area is treated. Loss of appetite, diarrhea and headache occur more
rarely. Proton beam therapy is also used in combination with other
cancer treatments that carry their own side effects.
Last summer, M.D. Anderson opened its $125 million proton
beam therapy facility following three years of construction and
testing, and Dr. Cox says the patients are flowing in. Officials
calculate the center can accommodate 3,500 cancer patients a year.
Today, cancer patients with the “right” kind of
tumor can receive proton beam therapy in six centers in the United
States: M.D. Anderson in Houston; Loma Linda University Medical
Center in California; the Midwest Proton Radiotherapy Institute
in Bloomington, Indiana; Massachusetts General Hospital in Boston;
the University of Florida Proton Therapy Institute in Jacksonville;
and the University of California Davis Cancer Center in Sacramento
(offered only for patients with ocular melanoma). There are more
than 20 proton beam therapy facilities elsewhere around the world,
and at least six other medical centers have plans to open the expensive
facilities in Philadelphia, West Chicago, St. Louis, Seattle, Oklahoma
City and Hampton, Virginia.
Popularity Boost
The Food and Drug Administration approved proton beam therapy
as a cancer treatment in 1988, and most medical insurers cover proton
beam treatment for specific cancers. So why all the fuss now?
Jerry Slater, MD, chair of the radiation department at Loma
Linda, which has been treating cancer with proton beams since 1990,
says the current excitement is a byproduct of better imaging techniques
that are finally accurate enough to let physicians take advantage
of the precision of proton beam therapy.
“You could have done proton therapy in the ’50s
if you had PET (positron emission tomography) and MRI (magnetic
resonance imaging),” Dr. Slater says, referring to two key
imaging methods. Those images let doctors understand the three-dimensional
structure of a tumor, which is necessary for guiding proton beams
to exactly the right place. “It’s also just such a high-tech
thing. There was a perception you couldn’t do something this
complicated in a hospital. We believed it could be done, and showed
it.”
Long-term clinical studies are finally being published that
demonstrate the effectiveness of proton beam therapy. Studies have
shown doctors can use proton beams to control ocular melanomas while
letting many patients retain vision; to control acoustic neuromas
while avoiding nerve injury; and improve survival rates from chordomas
and chondrosarcomas of the skull base, among many other results.
For the more common cancers, such as prostate cancer, studies
indicate proton beam therapy offers about the same tumor control
as X-ray radiation. Dr. Slater expects studies will soon demonstrate
improved survival. “Start with a better treatment and you’ll
get better outcomes,” he says. “There’s going
to be a lot more [data]. We’re still in our infancy.”
Six years ago, when Bob Marckini’s doctor called to
tell him he had prostate cancer, Loma Linda was the only medical
center in the United States offering proton beam therapy.
Marckini, 57, had just watched his brother go through surgery
for prostate cancer. “I was scared to death,” Marckini
says of seeing his brother in the recovery room after surgery. “He
was the patriarch of our family. Strong, healthy and athletic. I
watched him go through his slow recovery.”
With his own prostate cancer diagnosis, Marckini dug into
medical literature and sought advice from doctors and prostate cancer
patients who had undergone a variety of radiation therapies, including
conventional X-ray therapy, intensity-modulated radiation therapy
and brachytherapy. Although each therapy
seemed to have an equal chance of getting rid of the cancer, proton
beam therapy was non-invasive and by far promised the fewest side
effects. For instance, more than 60 percent of prostate cancer patients
suffer impotence after conventional radiation compared with 30 percent
after proton beam therapy.
Pediatric patients developmental stage makes
them particularly vulnerable to the side effects of radiation,
so researchers are excited about the possibilities of proton
beam therapy in children.
After 10 weeks of research, Marckini traveled from his home
in Massachusetts to California for proton beam treatment. He’s
now cancer-free and has created an online support community for
prostate cancer patients at www.protonbob.com. The only side effect
he experienced was temporary rectal bleeding after radiologists
irradiated the healthy cells in a thin margin around his prostate
to catch stray cancer cells. Since this area of the body houses
organs close together, some radiation can reach the inside rectal
wall.
Proton beam therapy is not appropriate for all cancer patients.
In a series of papers published in Acta Oncologica, Swedish doctors
estimated proton beam therapy would benefit “one in seven
patients for whom radiation therapy is indicated.”
Its power lies in its precision, so proton beam therapy typically
can’t help patients whose cancers have metastasized, or those
with diffuse tumors. But for isolated tumors, studies have shown
proton beam therapy is at least as powerful as its conventional
radiation counterpart without damage to sensitive structures near
a tumor. And because side effects often limit the ability to deliver
strong radiation doses by X-ray, proton beam therapy often means
more powerful doses can be delivered at one time, which may mean
fewer treatments.
Dr. Cox says proton beam therapy is especially promising for children.
Pediatric patients’ developmental stage makes them particularly
vulnerable to the side effects of radiation, including later cancers
induced by treatment, so researchers are excited about the possibilities
of proton beam therapy in children. The technique has shown tremendous
success in pediatric head and neck cancers, in particular, increasing
tumor control and survival.
Over the past decade, key improvements to conventional X-ray
therapy, specifically three-dimensional conformal X-rays and intensity-modulated
radiation therapy, have increased dose delivery to tumors and lessened
side effects. “The same things are going to happen with protons,” Dr.
Slater says. “Everybody’s goal is 100 percent of dose
delivery to the tumor, zero elsewhere.”
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