The Retinal Diagram Window
The Retinal Diagram window is where the shape of the tumor base and other anatomic landmarks of interest are digitized. Photographs of the inner surface of the eye can be displayed in this window to assist the digitization. This window is also where the height and shape of the tumor is entered, muscle insertions are estimated, the plaque is positioned and suture coordinates on the scleral surface are calculated. In fact, most of the decisions required in treatment planning are made here.
A retinal diagram is a 2D polar map of the retinal surface, in cartography it is referred to as an azimuthal equidistant projection. All points on such a map are at proportionately correct distances from the center point, and are also at the correct azimuth from the center point. The flag of the United Nations contains an example of a polar azimuthal equidistant projection with the north pole at its center.
The standard Posterior retinal map is centered at the posterior pole of the eye. The radial spokes are meridian lines which extend to the limbus. The circles, in expanding order, represent the equator, ora serrata, and the limbus. In Plaque Simulator, the standard retinal diagram is modeled as being an oblate spheroidal surface 1 mm inset from the outer surface of the sclera. The tic marks on the diagram are spaced at 1 mm of arc increments.
Plaque Simulator suture coordinates are a 2D coordinate system on the outer surface of the sclera. A suture coordinate consists of a meridian plane (analogous to earth longitude), expressed as either a clock hour or a toric axis marker angle, and a chord distance along that meridian, measured from the limbus circle. For example, in the illustration on the right, using the default clock hour method, the coordinates of the sutures are at (2:38,13.3 mm) and (4:09,12.8 mm). Using the optional axis marker method, those same coordinates would be expressed as the azimuthal angle and caliper distance pair (11°,13.3 mm) and (145°,12.8 mm). The Retinal Diagram Document prints both clock hour and angular versions on separate pages for reference during surgery.
The alternative Anterior map is centered on the pole of the oblate anterior surface of the eye as defined for the posterior projection. As in the posterior projection, this oblate surface is modeled as being 1 mm inset from the outer surface of the sclera from the posterior pole to the limbus and the curvature is then extrapolated from the limbus to the anterior pole.
The radial spokes are meridian lines which extend from the limbus to the posterior pole. The circles, in expanding order, represent the limbus, ora serrata, equator, macula, fovea and finally the posterior pole which distorts to form the outermost circle. The tic marks on the diagram are spaced at 1 mm of arc increments.
The anterior projection is used when working with iris and ciliary tumors.
In practice, the surgeon first marks the meridian lines on the sclera either from experience or by using a toric (azimuthal) axis marker, then, using a Castroviejo caliper, the suture points are marked at 13.3 and 12.8 mm from the limbus along the paired meridian line.
Toric axis marker