Purpose: This study describes how inconsistent thickness of Silastic inserts for eye plaque brachytherapy impacts target dose. Eye plaques are widely used for ocular melanomas as an alternative to enucleation, providing comparable local tumor control. A COMS gold-alloy plaque holds a 2mm thick Silastic insert, with each seed 1mm from the surface, which is assumed to be in contact with the sclera. However, Silastic inserts for the 22mm COMS plaque appear to be manufactured with variable thickness, unintentionally increasing distance between the source and prescription point.
Methods: 13 Silastic inserts, with a noticeable range of thicknesses, were measured for a 22mm COMS plaque using a caliper. BrachyVision treatment planning system was used for dose calculations, based on TG-43 formalism. A prescription dose of 8500cGy to the tumor apex was planned for a 22mm Iodine-125 plaque. Doses to critical structures (inner sclera, lens, macula, opposite retina, and optic disk) were determined along with percent prescription dose (%PD), relative to 8500cGy. All calculations were repeated accounting for variable Silastic thickness and hypothetical tumor apex heights of 5, 7, and 10mm.
Results: Silastic inserts were grouped into standard, medium, and thick based on mean measured thicknesses of 1.93 (n=5), 2.36 (n=2), and 3.22mm (n=6), respectively. For a 5mm tumor apex, the calculated %PD decreased from 100.0, to 92.6, to 85.8, for standard, medium, and thick inserts respectively. For other tumor apex heights, there was a similar reduction of %PD, approximately 7% per additional 0.5mm silastic thickness. These dose effects are more pronounced if heterogeneity corrections are accounted for, due to atomic number of Silastic (Zeff=11), compared to water (Zeff=7.4).
Conclusion: As Silastic insert thickness increases, it is important to address the consequential underdosage of the target. It is recommended that procedures are developed to measure and account for Silastic thickness.