RE: (extremity dose)

["Moroney, Roger W" <Roger.Moroney@mkg.com>]

Well, I guess I'll jump in now. We also "discouraged" direct handling of
unshielded activity, stressed external exposure reduction via time, distance
and shielding in training, but still, we had individuals who were coming in
direct contact with unshielded sources. I can't stress enough that, as John
Laferriere says, if the whole hand, including the ring, is in close contact
with the source, the ring will give a reasonable dose value. If the source
is small and contact is limited to the fingertips, then annual dose limits
can be exceeded quickly.

We have done some experiments, using TLD's, and Monte Carlo calculations for
some geometry's/sources. For example, contact dose rates, at 0.007 cm depth,
for In-111 (through a glass vial) are 1.59 rad/Ci/s (Monte Carlo). For those
of us who make In-111, there is also a substantial contribution from the
In-110b (4.9 h) impurity (10.6 rad/Ci/s). A one Ci vial of Tc-99m is good
for 120 mR/sec. We just did not recognize that the contact dose rates could
be that high or, that the repetitive nature of manufacturing and testing
would add up dose as fast as it did. The acute exposure which triggered this
investigation into contact handling resulted from a source with a dose rate
of 63 rad/sec (19 Ci Mo-99 and 8 Ci Tc-99m).

I would encourage everyone who handles more than a few mCi of H-3 a few
times a year to take a hard look at what your workers are doing. In our
case, most of the dose came a little bit at a time, but they do the same
process hundreds of times a year. Similar situations may exist at nuclear
pharmacies. Make sure remote handling tools are actually being used. Look at
the relationship between the ring to source distance and ring to closest
extremity distance. Remote handling tools, such as forceps and tongs, will
usually suffice but the workers have to use them. Over the last few months,
we have made an amazing variety of tools to hold and manipulate sources.

Landauer does make a fingertip TLD gizmo. It was tried out in a few of our
Nuclear Pharmacies but I'm told it wasn't very practical. Check with
Landauer for more information. While it didn't work for us, it might in a
different application. Radiopharmaceutical production is quite a bit more
intensive than anything else that I have seen. Not that I've been around
that long.

Now for the disclaimers:
Well, I won't put in many but I will say that I do not speak for
Mallinckrodt but as an HP who learned something the hard way. If someone can
learn from this situation, then I think it should be shared. As you can
imagine, the explanation is not something that can be reduced to a few
paragraphs so please don't look at the above as a complete, let alone
official, explanation. I'm sure most of you know how to find the NRC report
so you can look at that for more detail. There are a few errors and some
grandstanding in it but, hopefully, it might keep some one else from
repeating our mistakes. It would be nice to get what we have learned into a
paper or presentation. Right now, time is in short supply.

Roger Moroney
Health Physicist 
(contractor)
314.654.7457 voice
314-393-4562 cell
314.654.7998 fax
roger.moroney@mkg.com
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