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vilhelm_s t1_j7algoz wrote

Reply to comment by Tricky-Block4385 in A medical isotope made from nuclear weapons waste (Tc-99m) has a six-hour half-life. How do hospitals keep it in stock? by Gwaiian

Roughly how long is the cutoff time? (E.g. if the courier is late by 1 hour?)

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Tricky-Block4385 t1_j7b2bnj wrote

An hour wouldn’t make a difference typically, but each site has their own agreed-on dose ranges. At my sites, I have usually about a 2-3 hour range before my doses would be too low.

So the pharmacy you order from prepares the doses all together and calculates how much radiation will be in each dose at a prescribed time (so if I open a dose calibrated for 2pm at 9am, the dose is WAY too high to give to someone, but if I did give it, it would still work as intended). I’m giving this example in unit doses. Some places will order bulk doses, where they can draw up their own amount of radiation at any given time.

When I order for, let’s say three patients, we will say that each of them are coming in one hour apart from each other. I’ll order 10 mCi of technetium at 9 AM, 10 AM, and 11 AM. Each of my patients come at those times . If a patient is an hour late, the dose will be lower, but it will be usable still. After a certain time you are not giving enough radiation to make good pictures. So if my 9 AM patient comes at noon and I don’t have enough radiation to give them, I would reschedule them so they get better pictures a different day.

The other reason that those does expire is because the tag breaks up on the technetium. So when I do a patient, I’ll order 10 mCi of 99m technetium sestamibi, let’s say. The sestamibi will hold its tag to the technetium for several hours. If the pharmacy makes that dose at 2 AM and ships it out to me calibrated for 9 AM, I can inject the patient with that amount of radiation from probably 8 to 10 a.m. It’s around 5 PM or so, the tag is two broken up to still go to the correct place in the human body. Straight technetium 99m goes to the stomach, the thyroid, kidneys, and other areas of the body. Technetium 99m sestamibi goes to the heart muscle, but also goes to some of those other areas of the body too. So when the sestamibi breaks off, it won’t go to the heart muscle anymore. If I’m trying to get a heart scan on a patient, it doesn’t do me any good to inject something that isn’t going to go to the target organ anymore.

Does that make sense? I feel like I might be more confusing than helpful, but there are several reasons you wouldn’t inject after a specific time, not just that the radiation is decayed.

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