Re: (Fwd) Re: Lymphoscintigraphy

["Mike Bohan" <mike.bohan@yale.edu>]

        Reply to:   RE>>(Fwd) Re: Lymphoscintigraphy

Darren and RadSafers,

What would qualify to be trivial???

Let's see, if we take 1 mCi of Tc-99m and calculate the total dose to decay @ 0.5 meter:

Infinite Exposure @ 0.5 m = 

1.44 * 0.78 R-cm^2/mCi-hr * 1mCi/50 cm^2 * 6 hr T 1/2

I calculate 2.7 mR/procedure.  Of course, this completely ignores any self-shielding, any reasonable occupancy factor, and the fact that there is usually about 2-3 hrs between injection and the point of surgical removal.  Considering, a film badge threshold of 10% of the annual limits, the O.R. and Path. staffs would have to do more than 185 procedures/yr under this very conservative estimate to meet the whole body badging threshold.  

Using more realistic assumptions, I would estimate that the maximum activity at removal would be about 0.7 mCi, because you need to wait about 3 hrs post-injection before you start the surgery, and the O.R. procedure would take about 2 hrs, from start to tumor and node removal.

If we still assume a distance of 0.5 meters:

Exposure/case @ 0.5 m =

0.78 R-cm^2/mCi-hr * 0.7mCi/50 cm^2 * 2 hr

I calculate 0.4 mR/case to the maximally exposed surgeon or pathologist.

If we assume the Path. case takes 15 minutes, (The hands are actually in contact with the tissue specimens much shorter than this) with an average fingertip distance of 1 cm, and if we assume that they get the sample 4 hours post administration.  I calculate the following extremity exposure/case:

Extremity Exposure/case @ 0.01 m = 

0.78 R-cm^2/mCi-hr * 0.63 mCi/1 cm^2 * 0.25 hr

I calculate about 125 mR/case.  An individual pathologist would have to perform more than 40 cases/yr to exceed the extremity badging threshold.

The most important thing you can do it to train the O.R. and Path. staffs.  Give them realistic dose estimates/case and remind them that their universal precautions already protect them from contamination problems. If they use reasonable time, distance and shielding precautions, the doses will be minimal and the benefit to patients will be significant.

Consequently, with proper training and support from the radiation safety program, I believe that lymphscintigraphy guided node removals involve minimal exposures, if not trivial, from a radiation hygiene perspective. 

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Michael J. Bohan, RSO   |  e-mail: mike.bohan@yale.edu
Yale-New Haven Hospital |    Tele: (203) 688-2950
Radiological Physics    |     FAX: (203) 737-4252
20 York St. - WWW 204   |    As usual, everything I say may be plausibly
New Haven, CT    06504  |    denied at my employer's convenience ...
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