It was the worst news that the nuclear-medicine community could receive when, on August 12, Atomic Energy of Canada Ltd (AECL), Chalk River, Ontario, announced that the National Research Universal (NRU) reactor would remain shut down until at least January 2010. The 51-year-old reactor, which has been inoperative since May owing to a heavy-water leak, needs repair in at least nine areas, according to the AECL. The loss of the NRU reactor had already created a shortage of medical isotope technetium Tc 99m; when another reactor in the Netherlands was shut down on July 18, US supply of the radionuclide became severely limited.
“As physicians on the front line, we want the most appropriate test to answer the question at hand in as timely a manner as possible. There are only a handful of these reactors around the world, and they’re all aging. We’re having this problem now, we’ve had it before, and we will have it again in the future.”--Mylan Cohen, MD
Mylan Cohen, MD, president-elect of the American Society of Nuclear Cardiologists, characterizes technetium as “the workhorse of nuclear medicine.” The radionuclide, a product of the decay of molybdenum 99, is used in more than 40,000 imaging procedures daily in the United States.¹ “Technetium, when combined with various organic molecules, is used to target a number of organs to evaluate disease states,” Cohen says. “It’s used extensively in cardiology to evaluate blood flow to the heart, and it is also used to image cancer patients.”
Technetium has attained its workhorse status in nuclear medicine partly because of its short half-life (just six hours), which minimizes the dose of radiation received by the patient. That half-life is also a liability, preventing hospitals and imaging centers from stockpiling the isotope for future use. “This shortage points to the need for the United States to have a viable solution for future shortages, such as its own reactor,” Cohen says.
Rationing and Work-arounds
The very limited supply of technetium currently available to physicians in the United States has to be rationed carefully, Cohen says. “A trickle of technetium is coming from other reactors worldwide, but the supply is really scaled back. We have to pick and choose which patients will get it,” he notes. “It has a huge impact on the quality of the images.”
For nuclear cardiology, Cohen says, the most common substitution for technetium is thallium, which is cyclotron produced and has been widely used in the past. “Although it’s a well-validated radionuclide, there are a lot of characteristics of thallium that make it less preferable to use than technetium,” he explains. “It has a lower energy, so the images that we get with thallium aren’t as crisp.”
Using thallium can also lead to workflow issues, Cohen notes. “Thallium redistributes, so after you inject it, it goes to the heart, and then it moves around,” he says. “Technetium sticks to the myocardium like glue. Cardiac imaging with thallium means you have to alter your schedule to ensure that patients are imaged very quickly after injection.”
For patients with cancer, technetium is commonly used to image bone tumors or to locate the lymph node nearest to a breast tumor for biopsy. The alternative to using technetium to image breast cancer is injecting a dye, which is not as effective at pinpointing the node.¹ In some cases, PET with fludeoxyglucose F 18, also cyclotron produced, is being used for bone scintigraphy, but as Cohen says, “Alternative modalities aren’t always available in rural states like Maine. PET is highly sophisticated, and generally only offered in urban tertiary care centers.”
One factor that helps determine which patients get technetium is body weight, Cohen says. “Because technetium is a higher-energy radionuclide, it has higher penetration in larger patients. If we have a very large patient (over 250 pounds, which is not a rare occurrence in the United States), we may try to image that person with technetium.” In the meantime, Cohen says, some patients might get stress ECGs instead of technetium stress tests.
“There are alternative modalities for both patients with cancer and patients with heart disease, but I think what you try to do, as a physician, is obtain the appropriate test to answer the question at hand in the best fashion possible,” Cohen says. “The alternative to nuclear medicine can’t always do that. The shortage of technetium has meant that some patients who