In COMS style plaques, the radionuclide seeds are inserted into slots molded into the convex (back) side of a flexible silicone seed carrier. The slots are intended to offset the seeds by 1 mm from the concave (facial) surface of the plaque. The slots in the carrier are slightly longer and deeper than the seeds. This introduces a small uncertainty with regard to the seed position within a slot. For instance, depending upon the skill of the person assembling the plaque, or jostling during sterilzation, some seeds might end up slightly more than 1 mm from the face of the plaque.
The seed carrier is then inserted into the gold shell. Because the carrier of unnotched plaques can rotate within the shell, additional uncertainty is introduced. For instance, depending upon the skill of the person assembling the plaque, the rotational orientation of the carrier within the shell might vary by a few degrees from the intended plan. This flexible carrier system carries the potential for some uncertainty between the seed positions and the suture eyelets of the shell, and hence adds uncertainty to the calculation of dose to the tumor and critical structures within the eye. In general, this uncertainty is not dosimetrically significant with regard to the tumor, but might be significant at locations immediately adjacent to the plaque where the dose can depend upon the geometrical orientation of nearby seeds with respect to the collimating lip.
Plaque Simulator includes 3D models of many of the COMS style plaques. Plaque Simulator uses a polar coordinate system to keep track of the center of each seed, the rotation of each seed about its own center, and the rotation of the entire silicone carrier within the plaque shell. For comparison with other treatment planning systems, Cartesian coordinates for the centers of seed axis placeholders in the carrier slots are illustrated below. These coordinates assume a 0.8 mm diameter seed. For example, when a seed is fully inserted into its slot in the carrier, the cylindrical surface of the seed will be offset 1 mm from the face of the plaque. The offset to the seed center will be 1.4 mm.
Below are Plaque Simulator's coordinates based on the engineering models of the COMS plaques, followed by coordinates as determined from high resolution scans of actual samples of both standard and notched plaques.
When seed coordinates are displayed, the origin of the plaque coordinate system is shifted to the center of the concave surface of the COMS plaque's silicone seed carrier. The X axis of this coordinate system is the Central AXis (CAX) of the plaque. In the Plaque window you are looking at a 2D projection of the plaque onto the YZ plane.