The picture on the right compares the relative thickness of the ¹⁰⁶Ru applictors made by BEBIG, the ¹²⁵I (or ¹⁰³Pd) plaques used by the COMS, and the Eye Physics plaques.

The plaques designed for the COMS displace the ¹²⁵I seeds by 1 mm from the sclera. Increasing the distance of a radiation source from the treatment volume reduces the geometrical inverse square effect resulting in a more homogeneous dose distribution. By offseting the seeds, the ratio of dose to regions proximal to the tumor apex (such as the sclera underlying the tumor) compared to dose at the tumor apex is less than it would be if the seeds were closer to the eye. It also means that the relative dose to regions of the eye distal to the apex is slightly increased. The COMS approach also results in a rather thick plaque. Using distance to reduce the effects of inverse square is a good approach for a single seed.

Eye Physics plaques are thinner than the equivalent diameter COMS plaques and move the seeds physically closer to the eye. Because the distance between the seeds and tumor apex is less than in the COMS design, the inverse square effect is greater so dose to the distal eye is slightly reduced compared to the COMS plaque! Dose to the proximal portion of the eye is also generally less than the COMS design! How can this be?The Eye Physics plaques exploit the fact that there are MANY radiation sources and collimates each source to remove laterally directed primary radiation that would otherwise overlap and pass through the sclera or adjacent retina without contributing to the tumor dose. The collimated fields are designed to overlap just below the base of the tumor. This approach would not work for a single seed, but works well when there are many seeds. An analogy is external beam treatment using IMRT, CyberKnife or GammaKnife radiosurgery. None of these modalities would be effective given only a single beam. They also depend on a large number of highly collimated beams coming from many directions, all converging on the tumor.