Assembling a COMS Plaque

Plaques which use removable, short lived radionuclide sources such as the COMS, ROPES, and Eye Physics plaques can be assembled on-site about 24 hours prior to use, or can be ordered preloaded and sterilized from IsoAid. The radionuclide in BEBIG Ru-106 plaques is not removable so local assembly is not possible. On-site plaque assembly should be done in a hot lab behind an L block.


Prepare to load
  • The radioactive sources (or "seeds") are removed from the shipping vial.
  • A cardboard bowl is placed under the plaque to prevent losing the tiny seeds.
  • The seeds are handled using a long forceps.
  • A "third hand" device is used to stabilize the carrier while inserting seeds.

Load seeds into carrier

The seeds are inserted into the slots in the silicone carrier of a COMS plaque as illustrated, according to the treatment plan and loading diagram.


Glue the seeds?

The problem with using a silicone adhesive to secure the seeds in the COMS silicone carrier is that it is very difficult to remove the seeds from the carrier after treatment is completed because silicone adhesive can not be dissolved. The risk of not using any adhesive is that the seeds might dislodge from the slots resulting in loss or unpredictable dosimetry. At USC we were not satisfied with the COMS technology so we developed an alternative plaque design in the mid 1980s that later evolved into the Eye Physics collimated plaques.

When we did use the COMS plaques, we found that silicone adhesive was not absolutely necessary if the sources were fairly secure in the carrier to begin with. We placed a drop of cyanoacrylate adhesive on the seeds once they were secure in the slots. The cyanoacylate does not adhere well to the silicone carrier, but it does adhere to the metal shell of the seeds and forms a kind of plug that makes the seeds more resistant to falling out of the slots in the carrier.

A drop of medical device grade cyanoacrylate adhesive (e.g. Loctite 4014) can be used to coat the seeds in the slots. Use just enough glue to barely surround each seed.


Insert carrier into shell

When all the seeds are in place, place a few more drops of medical device grade cyanoacrylate adhesive onto the back of the carrier and then place the gold shell over the plaque with its suture eyelets oriented according to the loading diagram. The example on the right illustrates the different appearance of slots with seeds installed and those that are empty.

Let the cyanoacrylate dry overnight. If the humidity is very low, put a bowl of water near by. The cyanoacrylate glue hardens very quickly when trapped between two surfaces. The reaction is caused by the condensed water vapour on the surfaces. The water comes from the surrounding air, so the air humidity is a factor that affects bonding capabilities.

The curing reaction starts at the surface of the bonded material and develops towards the centre of the bond. Because of this, thick seams or large blobs of glue may harden less satisfactorily than surface-to-surface bonds with good fit. In a thick blob of glue, a polymerisation reaction may stop before it reaches the centre of the blob. A rule of thumb is that seams thicker than 0,25 mm should be avoided. Thick seams will also take longer time to cure.

To remove the glue, soak the plaque, seeds and carrier in a bowl of acetone for about an hour to disolve the glue.