To model a notched plaque:

• Create a model of the plaque in the Shell Editor window.
• Populate the model with source placeholders in the Slot Editor window.
• In the shell editor, select the perimeter node closest to the arced center of the notch (by clicking on it) and once selected, click the Mark Center button. This will tag the plaque model as being notched and initialize the circular centering tool. You may optionally mark nodes surrounding the notch center as being part of the notch group using the Lip Editor.
• Enter the desired diameter of the circle (in mm) in the text field and then drag the centering circle for best fit to the notch.

During centering and dragging operations, plaque models for which notch centers have been established will automatically balance themselves around the optic disc when the they come in close proximity to the disc. The dashed circle surrounding the optic disc in the retinal diagram approximates the myelin sheath surrounding the optic nerve and the closest approach to the disc that can be achieved without colliding with the nerve which will put pressure on the nerve and may cause the posterior edge of the plaque to lift away from the surface of the eye. You may also manually rotate a notched plaque to customize its fit around the nerve.

• Upper-left quadrant: COMS 14mm plaque
  
  • A uniformly loaded, standard COMS plaque with a circularly (ie a convex polygon) shaped lip can not get close enough to the disc to dosimetrically cover the tumor base.
  • The retinal dose-area histogram (RDAH) for this configuration indicates about 95% coverage of the tumor base by the Rx (e.g. 85 Gy) isodose line and about 75% coverage of the base + margin. A substantial area of univolved retina (plotted in blue as 'not margin') receives greater than 85 Gy.
• Upper-right quadrant: COMS 14mm plaque with notch
  
  • The notched COMS plaque, with its ringed seed pattern, only allows one seed to be positioned in close proximity to the disc.
  • The notched COMS plaque provides about 98% Rx coverage of the tumor base and 83% Rx coverage of the base + margin.
• Lower-left quadrant: EP917 plaque
  
  • The EP917 plaque uses shallow, wide-angle seed slots and a wide notch that brackets the disc with two tangentially oriented seeds.
  • The EP917 plaque, conformally loaded as illustrated, provides about 99% Rx coverage of the tumor base and 92% Rx coverage of the base + margin. Only a very small area of univolved retina (plotted in blue as 'not margin') receives greater than 85 Gy.
• Lower-right quadrant: EP930 plaque
  
  • The EP930 plaque uses shallow, wide-angle seed slots and a wide, deep notch that brackets the disc with two tangentially oriented seeds.
  • The EP930 plaque, conformally loaded as illustrated, provides 100% Rx coverage of both the tumor base and margin, but also delivers greater dose to the surface of disc.
  • The sutures are located close to the limbus, greatly simplifying the surgical procedure. The deep notch surrounding the optic nerve sheath assures that the posterior edge of the plaque will remain correctly positioned even though the sutures are located quite anteriorly.

The notches of these sample COMS plaques are all less than 4 mm wide. The ring shaped pattern of the seeds near the lip prevents adjacent seeds from straddling the disc and conforming to tumors that grow over and/or wrap around the disc.

In the three larger diameter plaques, only 1 seed lies close to, and is oriented tangentially to the disc. In the two smaller plaques, none of the seeds come close to the disc.

The orientation of the seeds with respect to the notch is inconsistent in this sampling. It is subject to variation depending upon where the notch is cut into the silicone carrier.

If the width of the notch is narrower than 5 mm, the notch will be a tight fit to the nerve. It will be more difficult to seat the posterior edge of the plaque near the disc and the sheath surrounding the optic nerve will be compressed by the notch.

Plaque Simulator detects and warns of notch-sheath collisions. Collision regions are illustrated with a red tint in the retina diagram.

The notches of Eye Physics plaques are wide enough to comfortably fit sheaths of up to 7 mm diameter. These models provide collimated seed positions which straddle the disc to conform to tumors that wrap around the disc while sparing the nerve.

With Eye Physics plaques, the location and orientation of all seeds with respect to the notch is consistent within a plaque model and therefore dosimetrically reproducible.

The notches of these Eye Physics plaques are wide enough to comfortably fit sheaths of up to 7 mm diameter.

The width of the notch at the perimeter of a plaque should be designed to accomodate the 5 to 7 mm diameter sheath that surrounds the optic nerve. The sheath limits how closely the plaque can approach the disc and this should be taken into account when planning.