Friday, December 14, 2007

Technetium-99 Shortage

My work involves being knowledgeable about both medicine and technology. Sometimes it allows me to totally get my geek on. Just yesterday I delved into the realm of nuclear chemistry to find out information about a worldwide shortage of technetium-99. You may heard about it in the news - it has been having an impact on the practice of nuclear medicine for the last couple of weeks.

A little history first (hey, I found this stuff out, so now you get to hear about it, too). Technetium, element 43, is the lightest element that has no stable isotope. That is, any form of technetium either already has or soon will decay into some other element. That is why is was not conclusively discovered until 1937. However, using the power of his invention, the periodic table, Dmitri Mendeleev was able to predict both its existence and some of its material properties decades in advance. Due to its location on the table, he figured it would be similar to manganese, and so called it ekamanganese. The reason it is called technetium today is because of its greek root, which means "artificial." Technetium was the first synthesized element - it was isolated from a molybdenum target used in early cyclotron experiments.

Nowadays, technetium is a workhorse in nuclear medicine. It is injected into patients in various forms and compounds, and emits radiation from the inside out that is used for bone density scans, determining the extent of a heart attack, and other diagnostics. The particular isotope, Tc-99m ('m' for metastable), undergoes a reshuffling of the protons and neutrons in its nucleus, settling into Tc-99, and spitting off a 140 keV gamma ray, which is soft enough to be picked up by X-ray equipment. The slightly more stable isotope, Tc-99, is a run-of-the-mill beta-emitter and gets excreted from the body pretty soon thereafter, limiting the total overall dosage. Tc-99 gets used in radiation calibration standards.

The decay from Tc-99m to Tc-99 has a half life of about 6 hours. That means that it doesn't stick around too long once it is made. Not long enough, even, for it to get transported. Its precursor, Molybdenum-99, however, has a half-life of nearly 3 days, which is ample time to transport it, extract the desired Tc-99m, and inject that into patients before it decays to diagnostically worthless Tc-99. Hospitals typically receive once weekly shipment of Mo-99 in what's called a technetium cow.

Working our way back up the supply chain, it is worth asking where all the Mo-99 comes from. As it turns out, nearly all the Mo-99 used in North America, and a fair portion of that used worldwide, comes from a single source: a heavy water nuclear reactor in Chalk River, Ontario. There are a handful of other sources worldwide, but Chalk River is the heavy lifter, and not only for Mo-99, but also other isotopes used for diagnostics and treatment.

As it happens, the reactor was shut down on Nov 18th for five days of scheduled maintenance. Once they got started, however, they realized there was other maintenance that they really ought to do then, too. So, rather than five days of downtime (roughly two half-lives of Mo-99), the hiccup stretched into weeks. The projected restart for the reactor had gone out as far as mid-January. That works out to about 23 half-lives of Mo-99, meaning that 1 gram of Chalk River Mo-99 that was out there in mid-November would have decayed to about 0.1 micrograms of Mo-99, a few nanograms of Tc-99m, and 0.999 999 grams of Tc-99.

To put it another way, the extended downtime of the reactor meant that the supply of Tc-99m was inexorably drying up (or, more specifically, decaying). Effects varied here and there, but by and large it meant that some tests were delayed, patients were put in a priority queue, and diagnostics were done with lower dosages (and hence, yielded poorer results). By some accounts, it was a mounting crisis.

Enter the Canadian Parliament and the Prime Minister Stephen Harper. Yesterday, as I was doing this research, the Parliament passed legislation ordering the reactor to start up again. The startup was ahead of schedule and against the advice of Canada's nuclear regulators. Still, a crisis is a crisis.

With the reactor ramping up again, the production of medical isotopes and reach full capacity within a week.

I wish I could come up with a nifty epilogue to this story, but that's about all I've got. I suppose this even could strengthen calls for additional redundancy in the supply chain - create the ability at other nuclear reactors for medical isotope production. If nothing else, with was a fun bit of research in an otherwise ordinary day.

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