[ RadSafe ] " Short life of an isotope "
jaro-10kbq at sympatico.ca
Sat Jul 4 09:16:18 CDT 2009
Short life of an isotope
A "For Rent" sign stands in front of a nondescript building in Dorval's
industrial park, giving no hint of the life-saving work going on inside.
By Aaron Derfel, The Gazette, July 4, 2009
The building's windows are tinted and the front door is locked. Yet behind
the walls, Geiger counters are crackling as technicians prepare doses of
radioactive medical isotopes to help doctors diagnose cancer and heart
The radiopharmacy runs seven days a week, 24 hours a day, supplying most of
Montreal's hospitals, but its owners want to keep the location a secret.
"We don't want any publicity," said Cyrille Villeneuve, vice-president
(international) of Lantheus Medical Imaging. "You know, there are activists
and we have radioactive materials." A single dose of medical isotopes is no
more radioactive than a chest X-ray. But the technicians at the Dorval
radiopharmacy handle generators buzzing with isotopes that are hundreds of
times more radioactive than what is in injected into a patient's arm.
Still, "it's not something to be scared of - working with radiation," said
Sacha Des Serres, a technician with a meticulous manner and an easy smile.
"We're well protected," she added, showing off a dosemeter ring, which
measures her potential radiation exposure. "We stand behind lead shields and
everybody works well together." Unlike the corner pharmacy, the Lantheus
facility does not stock drugs with long-term expiry dates. Medical isotopes,
which course through the body as tracers to diagnose disease and to treat
some forms of cancer, cannot be stockpiled. You can't see these isotopes
with your own eyes, smell them or taste them because they're nothing more
than a small collection of radioactive atoms.
Their sole purpose is to emit gamma rays from internal organs for scanning
by medical-imaging cameras.
As if they weren't elusive enough, the most common isotope employed in
nuclear medicine - technetium-99m - has a half-life of only six hours.
Derived from the Greek word "technikos," meaning "artificial,"
technetium-99m doesn't even exist in nature. It's forged partly in a nuclear
reactor, and after six hours, half of a dose disappears, and after another
six hours, half of what remains vanishes as well, and so on.
Therefore, time is of the essence when dealing with medical isotopes, and
that's why a radiopharmacy operates 24 hours a day. But since the end of
May, when Ontario's aging Chalk River nuclear reactor shut down because of a
leak, isotopes have been in short supply across North America.
Quebec hospitals have put off at least 12,000 diagnostic tests and have
delayed the treatment of some patients with thyroid cancers. The Dorval
radiopharmacy has resorted to ordering isotopes that are manufactured by
nuclear reactors in Europe and South Africa.
By plane and by truck, the isotopes arrive at the Dorval facility each day
in boxes marked with the stark tri-blade radiation symbol. Inside those
boxes sit 300-pound, lead-encased isotope generators, or cows, as the
technicians prefer to call them. To the untrained eye, a generator could be
mistaken for a milk bucket.
In the unseen core of each generator is a pencil-sized column of alumina
powder. It's the alumina that absorbs the isotopes - in this case,
molybdenum-99. Moly-99 is what is actually created in the nuclear reactor.
It has a half-life of 66 hours, making the isotope ideal at this stage for
transport in trans-Atlantic flights to radiopharmacies.
As Moly-99 decays inside the generator, it gives birth to a daughter - yes,
daughter is the word nuclear physicists use - named technetium-99m, or
Tc-99m for short. The generator is good for a week before it runs out of
most of its isotopes.
So what Des Serres and her colleagues do is "milk" the "moly cow" for
technetium. Actually, the process is quite delicate: It involves infusing a
saline solution into the column in the generator and rinsing Tc-99m from the
alumina. The isotope, in a clear liquid, is milked out of the top of the
generator into a vacuum-pressurized vial.
Since Moly-99 is not soluble, it stays behind in the generator, continuously
decaying into technetium.
Usually, a technetium generator is milked once or twice a day. But given
that the Chalk River reactor is down and had met more than 30 per cent of
the world's isotope needs, radiopharmacy technicians are now milking their
moly cows up to four times daily. This poses a problem, because the
technicians must also squeeze in the time to prepare doses in syringes.
What's more, quality-control technicians have to double-check the technetium
for impurities right after it's milked from the moly cow as well as the
doses before they are shipped to hospitals.
"It's more trouble," said Richard Dubois, manager of the Dorval
"We're doing the best we can but we're limited by the amount of raw material
we get. As a result, we have to work much harder just to produce the same
number of doses."
Standing behind a lead shield, Des Serres started to fill an isotope
prescription for a patient who was only known to her as a bar code on a
label. She peered through a thick lead glass window as she stuck the needle
of a syringe into the top of a vial in a lead container, slowly drawing out
the technetium. The Geiger counters clicked in the background; if they
suddenly screeched, that would have signaled a major radiation leak.
Depending on the purpose of the dose - whether to study heart function or to
determine whether a cancer has spread to the bone - the technetium is
"tagged" to a chemical compound. The syringes are then encapsulated in
colour-coded lead tubes, and placed inside virtually indestructible lead
suitcase. Drivers waiting outside transport the suitcases in unmarked vans
to hospitals. To make sure the patient receives enough of a dose, Des Serres
will boost the amount of initial radiation in the syringe to take into
account transport time and technetium's short half-life.
"If we can free up the hospitals by making these doses, that's a good
thing," she said.
Ironically, the one hospital in Montreal that doesn't receive doses from a
radiopharmacy is not suffering from a lack of isotopes. In fact, Jean Talon
Hospital is sending some of its excess isotopes to other local hospitals.
Years ago, Jean Talon signed a contract with a U.S. supplier for weekly
shipments of technetium generators. Each weekday at the community hospital
in Villeray, a technician milks the generator and readies the doses.
"Since we do everything at our hospital, there is no loss of isotopes,"
explained André Arsenault, Jean Talon's chief of nuclear medicine. "When the
isotopes are prepared at a radiopharmacy at 5 a.m., by the time they arrive
at our hospital, half the doses are gone because of their short half-life."
This is not to suggest that the isotope shortage could be resolved if every
hospital was furnished with its own generators. The crisis came about in the
first place because of Chalk River's breakdown, not because of any problems
in the distribution network. And Jean Talon, unlike some of the big teaching
hospitals, doesn't have a huge volume of patients.
But the pony-tailed Arsenault, who has a strong independent streak and likes
to keep a close watch on things in his department, prefers to have a
technetium generator on site.
On a rainy June morning, Arsenault was reviewing the medical file of a
patient, 73-year-old Aimé Brunelle, who was about to undergo an isotope
diagnostic scan of his heart.
Two days earlier, a gamma camera scanned Brunelle's heart while he was at
rest. And now Brunelle had returned to undergo a second scan during a stress
test. Since Brunelle weighs more than 350 pounds, Arsenault decided against
having him run on a treadmill.
A nurse injected him instead with Persantin to boost his heart rate.
Brunelle lay on a stretcher, and after only a couple of minutes, his
breathing grew laboured.
"How are you doing?" Arsenault asked.
"I'm fine," Brunelle huffed. "Never been better."
The nurse then injected him with Myoview, a cardiac imaging agent that is
tagged with technetium. The agent zeroes in on the heart, with the isotopes
tagging along for the ride.
Once the technetium reaches the heart, it emits gamma rays. Unlike an X-ray,
which is beamed from a machine outside into the body, isotopes shoot rays
from within organs like the heart.
X-rays take detailed images of fractured bones. MRI and PET scans reveal
tumours swelling inside organs. By comparison, medical isotopes are used to
analyze organ function and can even predict the onset of disease. And that's
what makes them so indispensible to modern medicine.
A half hour later, Brunelle lay on a bed as it slid under a huge,
multi-million-dollar gamma camera. The machine took 20-second images of his
heart from different angles, a computer recreating a 3-D image of the organ.
Later that morning in his office, Arsenault called up the images of
Brunelle's heart on his computer screen.
"For a big man, he has a small heart," Arseneault said, studying the
"before" and "after" pictures.
He clicked on his mouse, calling up a 3-D computerized animation of
Brunelle's heart beating. This time, the "before" and "after" images were
much easier for the layman to understand, highlighting in orange and red the
heart pumping blood.
"His heart's fine," Arsenault concluded. "There's not much difference
between the two images."
Brunelle, a jovial snow-removal operator with 50 years' experience, had
already left the hospital. He would receive some good news later that day -
thanks, in part, to a daughter named technetium-99m.
- - -
>From Reactor to Hospital
Swords into ploughshares: some of the highly enriched uranium from
decommissioned U.S. nuclear warheads is sent to Chalk River to make medical
Chalk River's National Research Universal reactor was shut down for a
three-month repair at the end of May, and is to be phased out by 2016. For
the purposes of this graphic, Chalk River is used as an example.
A uranium-aluminum alloy no bigger than a highlighter marker is fixed as a
target in the nuclear reactor. It's bombarded with neutrons for five to
seven days. Through fission, radioactive molybdenum-99 is created, along
with other isotopes.
The irradiated target is cooled for a half a day, and undergoes first-stage
processing at Chalk River. In liquid form, it's transported in a shielded
cask to a "hot cell" at the nearby MDS Nordion facility for final processing
for up to 19 hours. Workers operating remote manipulators treat the target
with chemicals to recover pure Moly-99.
The Moly-99 is sent by plane to a plant in Billerica, close to Boston, to be
incorporated into lead-shielded isotope generators.
The generators are shipped to radiopharmacies and some hospitals across
Canada and the U.S. Technicians extract from the generators the daughter
radioisotope of Moly-99, technetium-99m.
Time is critical because of the short halflife of Moly-99: Less than 24
hours elapse from the moment it's purified at MDS Nordium until a technetium
generator is made in Billerica.
About 97 per cent of the highly enriched uranium that is used in making
medical isotopes ends up as waste that must be stored. It has a half-life of
700 million years.
Sources: Reporter interviews, U.S. Nuclear Radiation and Studies Board,
Lantheus Medical Imaging
aderfel at thegazette.canwest.com
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