Dose Reconstruction – What Happens if a Rad Workers Loses A Dosimeter?
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Dose Reconstruction – What Happens if a Rad Workers Loses A Dosimeter?

By Dr. Zoomie

Dear Dr. Zoomie – one of my rad workers lost his dosimeter. He normally gets some radiation exposure from working with our equipment – some gives off x-rays and some uses radioactive sources. What should I do?

From the information that you’ve provided I think this is something that has to be taken care of sooner rather than later – having said that I’m guessing that this is important rather than urgent. In other words, you have to make sure you meet regulatory requirements, but the worker’s health is probably not at risk (at least, based on the fact that you said the worker normally gets exposure, but you didn’t say that he normally is close to dose limits – and there’s a huge gap between reaching a dose limit and facing potential health effects). So – if my assumptions are correct – what you need to do is to come up with a reasonable dose estimate that you can provide to your dosimetry vendor so you can ask them to assign that dose to the worker in his dosimetry record. Here’s how you can go performing a dose estimate. And remember – make sure you document everything!

What I’m going to do is to lay out a bunch of techniques you can use – in some cases, a single one might do the trick; in other cases you might have to try several or even all of these.

One thing you can do is to look at the worker’s past dosimetry reports to see what level of exposure he’s received in the past. If, for example, his exposure has typically been between 50-100 mrem monthly AND if his workload for the month with missing dosimetry was typical then you can make a safe guess that, during the month in question, he probably received no less than 50 mrem and no more than 100 mrem. In this case, as the RSO, I could justify assigning a dose of 100 mrem to the worker for the month. But it would be nice (if possible) to come up with a second estimate to justify the first. So what else can you do?

You didn’t say anything about co-workers, but if you have more than just this one person who does this sort of work, you can check to see what dose his colleagues received during the moth with missing dosimetry. If their doses are normally comparable to the dose of the man with the missing dosimetry then you can see how much exposure they received and use that as the basis for a dose assignment. Say you have three other rad workers who received 50, 80, and 60 mrem for the month in question – being conservative, you can assign a dose of 80 mrem to the worker.

You have yet another option – and this one will require a little more work, but it’s likely to be most accurate. First, take a look at what the worker did during the month in question. For example, if he’s a radiographer you’re going to have to see how many jobs he went out on, which source he used for each job, and how many shots each job entailed. Or if he’s a nuclear medicine technologist, you need to see how many patients he saw and how much dose each received. Once you’ve figured out what the worker did you can start making radiation measurements. For example, if the worker did three radiography jobs with a 50 Ci source of Co-60, you can set up a mock radiography shot that mimics the jobsites he would have worked (in a safe location). Then you can perform a dummy shot in which you run the source out and retract it while you measure the radiation exposure – this will tell you how much dose your worker received during a single shot; this can be used to calculate his total dose for ALL of the short he performed during that month. For example, if you measure a dose of 5 mrem for one shot and he performed a total of 15 shots during the month you can assign a dose of 75 mrem for his radiography duties for that month. Alternately, you can perform this same measurement on a real shot and use the results to make the same calculation.

There’s another way to accomplish this, say, for someone who works in radiation areas or around radioactive materials. In this case, you need to try to estimate how much time the worker spent in each radiation area and then go to these areas yourself to measure the radiation levels. Say (for example) he spent a half hour performing sealed source leak tests – you’d need to go to your source storage and measure radiation levels. If you measure 20 mR/hr then you multiply this by a half hour and determine that his source leak tests gave him a dose of 10 mrem. Repeat this for every other task he performed and add these numbers together – this is the worker’s assigned dose for that month.

If time permits – and if the estimated dose is fairly high (more than 10% of a dose limit is my general rule of thumb, although others will do this for any calculated dose greater than 100 mrem) – you should do more than one technique and compare the results. Say the worker’s typical dose is 50-100 mrem monthly for the last several years, that his co-workers received doses of 50-80 mrem for the same month, and your measurements suggest a dose of 120 mrem for that month. These are all in the general ballpark – close enough that you could justify assigning a dose of anywhere between 80-120 mrem. If you want to use the average you’d pick 100 mrem, but I’d be a little more conservative and would likely go with 120 mrem. Where you have to really think, however, is if there’s a wide disparity – say your calculations, instead of showing 120 mrem, showed he might have received 3000 mrem during that month? You can always go with the measurements and assign a dose of 3000 mrem, but I’d want to do some more checking first. In this case, I’d re-run the dose measurements and calculations to see if I made a mistake. If my measurements and math seem to work out, I might even call a consultant to do an independent dose assessment as well as to review the work I’d done to see if these numbers could somehow be reconciled. You might have to go through this process a few times until you’ve got an answer you can feel comfortable about – once you reach this point you’ll need to contact your dosimetry vendor to ask them to assign this dose to the worker for that month. Oh – something to remember – if you base any part of your dose assignment on radiation surveys you performed, you need to file your survey results away with your dosimetry records and then hang onto them for as long as your company has a radioactive materials license.

There’s one related topic I’d like to mention before signing off – what to do if the dosimetry report comes back showing a dose that seems way too high. Say a worker’s badge reads 45 rem (45,000 mrem) one month. This is a high dose – not dangerously high, but far higher than the worker’s annual dose limit. Unless the worker was responding to a radiological emergency there’s no acceptable reason to have so high a dose – you need to try to figure out if the dose is real. With some dosimeters, you can ask for an assessment to see if the badge was attached to the worker during the exposure (this is called “static/dynamic imaging” by one vendor). But this should be verified by a dose reconstruction – you should do everything described above: compare his dose with previous months, compare his dose with co-workers, look at his work schedule, AND make dose measurements in all the areas where he worked (and under the same conditions under which he was exposed); maybe consider calling in a consultant as well.

But with a dose this high you have an option that a lower dose doesn’t give you – you can send a blood sample off for biodosimetry; most likely a procedure called chromosome aberration analysis. This looks at the chromosomes to see if they’ve been damaged by the radiation; if so, the amount of damage can be used to estimate the dose. The body is the ultimate arbiter of dose – no matter how high a dose the dosimeter shows, if the body shows it received a small dose then the dosimeter must be wrong.

OK – having said all of this, I have to acknowledge that you can get even deeper into dose reconstruction than this, but if what I’ve described above doesn’t solve your problem then you really need to bring in a consultant to help you out. In addition, if your worker had any sort of an uptake (inhalation or ingestion) then you should really consider bringing a consultant in as soon as possible. But barring one of these possibilities, this should stand you in good stead. Good luck!