Although one may not think
of the heart as a target of radiotherapy, several studies done in the
past few years suggest that a special form of irradiation, brachytherapy
(pronounced "bray-key-therapy"), may be a way of extending benefit
from balloon angioplasty, an already successful treatment for blocked
coronary arteries.
What is balloon angioplasty?
Angioplasty involves passing
a catheter into the blocked artery and inflating a tiny balloon attached
to its tip to open the vessel and restore free blood flow (with its supply
of oxygen) to the heart wall, either after a heart attack or when one
is threatened. Starting in the late 1970s, it has increasingly replaced
coronary bypass surgery, which requires a heart-lung machine and is a
much more invasive procedure. Today roughly 800,000 patients have angioplasty
each year, and a majority also have a stent—a wire mesh tube or coil—placed in the artery after it has been widened, to act as a sort of
scaffold and hold it open. In up to one in five of these patients, the
artery will become narrowed, or stenotic, again in a few months—a condition
called restenosis. Many of these patients will have to undergo repeat
angioplasty or bypass surgery. In fact, angioplasty may have to be repeated
more than once. There are an estimated 150,000 patients with restenosis
despite stent placement who could potentially benefit from brachytherapy,
a comparatively simple and quite safe approach.
What is brachytherapy?
Brachytherapy consists of placing
a source of radioactivity close to or implanted within the tissue to be
treated; the best-known example is placing radioactive "seeds"
within a cancerous prostate gland. Brachytherapy actually means
"near radiation," as opposed to the more common teletherapy
("far radiation") where the radiation source is some distance
from its target. After a cardiologist completes a repeat angioplasty and
after inserting a new stent, an interventional radiologist uses a catheter
to place a ribbon of radioactive seeds—each the size of a grain of rice—into the diseased artery for three to 20 minutes, and then removes it.
The procedure time varies depending on the type of system being used and
the planned dose.
Radiation is effective in preventing scar tissue
After the angioplasty balloon
has been inflated to stretch the artery wall, scar tissue may form and
reduce the caliber of the vessel wall, just as arteriosclerotic disease
did in the first place. This scar tissue may represent an attempt by the
artery wall to heal itself after microscopic damage caused by the inflated
balloon or the presence of a stent. Often, however, it restores the danger
that angioplasty was intended to avoid—that coronary artery blood flow
may in time stop altogether, causing a heart attack. The risk is greatest
for patients having small coronary arteries or long or multiple lesions
as well as those with diabetes. Radiation has long been recognized as
an effective way to prevent excessive scar tissue from forming. Cells
that divide rapidly—like those making up scar tissue—are sensitive
to radiation. The virtue of brachytherapy is that, with the help of diagnostic
x-rays taken after injecting contrast material (angiogram),
a precisely controlled amount of radiation may be delivered to the exact
site of artery damage.
How coronary brachytherapy is performed
Late in the year 2000, the
Food and Drug Administration (FDA) approved two devices for delivering
intravascular coronary brachytherapy. One emits gamma rays from
an element called Iridium, and the other, beta rays from radioactive
Strontium. Gamma radiation easily and more evenly penetrates
the target tissue, but takes longer to deliver (about 20 minutes) and
requires staff to take special precautions to avoid being exposed. In
contrast, beta radiation takes only three to five minutes to
deliver and staff may remain in the cath lab, avoiding exposure. Beta
radiation has limited penetration and is approved only for lesions up
to 20 mm long (.78 inches), whereas gamma radiation can eliminate lesions
45 mm long (1.77 inches). The procedures are still being refined, especially
to precisely deliver radiation therapy to ensure that the entire artery
wall is adequately treated.
Brachytherapy adds from 10
to 30 minutes to angioplasty. The FDA requires that it be done by a team
that includes a cardiologist experienced in interventional procedures,
a medical physicist, and a radiation oncologist (cancer specialist) who
is expert in safely delivering exact amounts of radiation. The treatment
already is offered at over 100 centers throughout the United States.
Current trials/results of brachytherapy
Vascular brachytherapy has
been under intense investigation. In the past five years, more than 7,000
patients have taken part in over 50 FDA-approved trials, and at least
a dozen trials are in progress. The results to date show that, when a
stent is in place, vascular brachytherapy using either gamma or
beta radiation lowers the risk of recurrent restenosis by 40
percent to 70 percent. Apart from preventing further restenosis, can brachytherapy
prevent restenosis when applied at the time of a first angioplasty? Unfortunately
the answer from a recently reported study is negative, but this may be
related to the length of the radiation source which was used.
No complications have occurred
in patients followed for five years after brachytherapy. Those close to
the patient are in absolutely no danger of radiation exposure. There is
an increased risk that blood will clot when a stent is replaced at the
time of radiation, but this danger has been addressed by placing another
stent only when absolutely needed and by giving patients a blood-thinning
drug for many months. Many more years of follow-up will be needed before
ruling out late radiation effects such as arterial aneurysm (weakening
of the arterial wall, which balloons out and may rupture) and cancer,
but these risks are expected to be very small or nonexistent.
Some benefits of brachytherapy
Although brachytherapy adds
to the expense of angioplasty, in the long run it is expected to lower
costs by avoiding—or at least delaying—repeat angioplasties. Patients
with coronary artery disease will not have to return to the hospital frequently
for retreatment. There is no harm in repeating brachytherapy, although
any previously treated area of artery should be avoided. Dr. Prabhakar
Tripuraneni, a radiation oncologist at the Scripps Clinic in La Jolla,
California, enumerates the potential benefits of vascular brachytherapy
as providing "an increased quality of life, less radiation exposure
to staff and patients in the long run by avoiding repeat procedures, and
millions of healthcare dollars saved."
The future of brachytherapy versus bypass surgery
Apart from preventing restenosis
of coronary arteries, vascular brachytherapy should help reduce narrowing
of arteriosclerotic arteries supplying the legs when angioplasty fails
to give long-term benefit. Questions regarding whether brachytherapy will
further tip the balance favoring angioplasty over bypass surgery and what
issues this may raise between surgeons and radiologists cannot yet be
answered. However, Dr. Tripuraneni believes that "the pendulum is
swinging in favor of angioplasty with its results improved by adding brachytherapy
in selected patients."
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