Notched Plaques

Using Notched Plaques

When the perimeter of the tumor overlaps, or lies adjacent to the optic disc, a plaque with a notched perimeter can provide more conformal dosimetric coverage of the tumor base and its surrounding margin. The improvements in dosimetric coverage of the tumor result from allowing the radionuclide seeds to move closer to the disc than would otherwise be possible in a plaque with an ovoid perimeter and, in some plaque designs, such as the Eye Physics model 930, enabling a pair of seeds to straddle the disc.

The examples that follow illustrate a small tumor that slightly overlaps the optic disc. A dose of 85 Gy has been prescribed to a height of 5 mm above the tumor apex in order to provide dosimetric coverage of the tumor base and a 2mm surrounding margin. The plaques have been positioned as close to the disc as possible assuming that the dural sheath surrounding the optic nerve has a diameter of 5 mm.

Uniformly loaded circular COMS plaque

The 85 Gy isodose line of a uniformly loaded standard circular COMS plaque does not fully enclose the tumor base and irradiates a large surface of univolved retina. Dose to the macula is about 30 Gy.

Partially loaded circular COMS plaque

The 85 Gy isodose line of a posteriorly loaded standard circular COMS plaque using 4 very high intensity seeds barely encloses the tumor perimeter and irradiates less uninvolved retina, but to a somewhat higher dose compared to the uniformly loaded COMS plaque. Dose to the macula is greater than 30 Gy.

Conformally loaded notched Eye Physics plaque

The 85 Gy isodose line of a conformally loaded notched, semi-elliptical Eye Physics plaque uses lower intensity seeds and encloses the tumor perimeter and margin while delivering lower dose to the underlying sclera and to the critical uninvolved retina between the macula and disc.

Optic nerve diameter

When implanting a notched plaque, the surgical procedure is easier and placement more secure when the notch is a good fit to the sheath that surrounds the optic nerve.

The diameter of the dural sheath surrounding the optic nerve where it connects to the sclera ranges from 5 to 7 mm.

COMS plaque notches

The notches of these sample COMS plaques are all less than 4 mm wide, and the seeds can not straddle the disc to conform to tumors that wrap around the disc.

If the width of the notch is too narrow, the plaque will be difficult to position around the nerve and the sheath surrounding the optic nerve will be compressed by the notch.

Eye Physics notches

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

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