[ RadSafe ] Nuclear Cleanup Could Derail an Experimental Cancer Treatment, Study Says

[ROY HERREN]

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June 4, 2008
Nuclear Cleanup Could Derail an Experimental Cancer Treatment, Study Says 
By MATTHEW L. WALD
WASHINGTON — A cleanup of nuclear waste from the cold war era threatens to eliminate the supply of an obscure isotope that shows great promise in cancer treatments, according to a report by the Energy Department’s inspector general.
But the department has concluded that the material, uranium 233, which does not exist in nature, is too expensive and risky to keep. The dispute is a rare instance where environmental cleanup and human health are in direct opposition. 
“The department is poised to dispose of a national resource,” said the report, which was released on Monday.
Most of the material was produced by reactors at the Oak Ridge National Laboratory in Tennessee by irradiating a naturally occurring material, thorium, but those reactors no longer operate. A smaller amount is stored at the Idaho National Laboratory, in Idaho Falls. The Idaho laboratory has a test reactor that could produce uranium 233, but replacing the 700 pounds already stored there would take about 1,000 years, the report said.
Complicating the situation is that uranium 233 can be used as fuel in nuclear weapons, meaning it cannot be commercially distributed, and can be handled only in small quantities, to avoid the risk of an unwanted chain reaction.
Produced by adding an extra neutron to thorium, the uranium slowly breaks down into another form of thorium, and then into other isotopes, including actinium 225 and bismuth 213. What distinguishes both of these for cancer treatments is that as they decay into new materials, they emit alpha particles. These particles can be superior to the standard form of radiation used to treat cancer, gamma rays, because the rays travel long distances through tissue and damage many cells, while the alpha particles have very short trajectories, and carry relatively huge amounts of energy.
“A single atom delivered to a cancer cell can kill that cell,” said Dr. David A. Scheinberg, chairman of the experimental therapeutics center at Memorial Sloan-Kettering Cancer Center in New York. “Nothing else approaches that.” 
Research trials at Sloan-Kettering on patients with acute myeloid leukemia are showing promising results, Dr. Scheinberg said, using antibodies to deliver the radioactive atoms directly to the tumors. Researchers there are testing the technique in animals for prostate, colon, lymphoma and brain tumors. 
The isotopes have a convoluted history. Uranium 233 very slowly breaks down into thorium 229, half of it making the conversion over 159,000 years. There is a significant amount of thorium 229 only because the Energy Department has had tons of uranium sitting around for decades. 
The thorium is not medically useful, but its half-life, the time for half to convert, is 7,340 years, and it decays into a radium isotope, and then into actinium, which has a 10-day half-life. (Short half-lives are desirable in this kind of cancer treatment, because such materials deliver their dose promptly.) One of the “daughters” of actinium is bismuth, which has a half-life of only 45.6 minutes. Researchers are testing both the actinium and the bismuth as therapeutic drugs.
The bismuth, in turn, decays to a stable isotope, with “acceptable toxicity,” according to the research.
Various alternative processes have been proposed for making actinium and bismuth, but none have been proved. 
The Food and Drug Administration has approved two radioactive materials for delivery by antibody, both beta emitters, Dr. Scheinberg said; it has not yet approved an alpha emitter. Beta particles have less energy and travel farther, making them less helpful in treatment. 
The Energy Department spends $5 million a year on security at the building at the Oak Ridge National Laboratory where most of the uranium 233 is stored, and the building needs a one-time investment of about $25 million if it is not retired soon, the agency told the inspector general. 
In 2005, Congress, concerned about security and safety, told the Energy Department to dispose of the material; the plan is to dilute it with depleted uranium, cast the mixture in cement and bury it at an underground repository near Carlsbad, N.M. But shipments would not begin for another few years. 
“It is the highest-security building we have down at the Oak Ridge facility right now,” said Frank Marcinowski, a deputy assistant secretary of energy. “That’s one of the reasons why we’re behind Congress’s direction for us to disposition it.”
Allen G. Croff, a retired Oak Ridge executive who served on a National Academy of Sciences panel on the cleanup of radioactive materials, said in an e-mail message, “In essence, this is yet another decision concerning federal budget priorities.” The current conclusion, Mr. Croff said, is that maintaining the stockpile “is not worthwhile,” but he said it might be time for an independent review.