Dosimetric quality of partially loaded I-125 COMS eye plaques for practical placement on posterior tumors

Eye malignancies that are suitable candidates for low dose-rate brachytherapy have been treated using the Collaborative Ocular Melanoma Study (COMS) eye plaques with radioactive I-125 seeds at the University of Oklahoma Medical Center (OUMC). Partially loaded plaques are used to make surgical placement more convenient, especially for tumors that are posteriorly located. Over the past years, clinical cases have been performed using posterior loaded 16 mm plaques. In posterior loaded plaque cases, the prescription point is shifted off-axis towards the center of the actual seed distribution; this allows normalization of the dose to a desired depth and area of coverage. This study investigated the dosimetry of a 16 mm fully loaded eye plaque and partial loads equivalents (approximated) to 12 mm and 10 mm eye plaques with radioactive I-125, 6711 model seeds. The dose calculation agreement was assessed for two commercially used treatment plans and verified with film measurements. The standard treatment plans were calculated using the Nucletron Planning Systems (NPS) and compared with plans generated with Brachtherapy Planning 11.0.47 (Varian Medical Systems). Delivered doses were measured in water using Gafchromic EBT3 film. Doses at the prescription point as well as dose to points corresponding to critical structures were evaluated. The findings of this study reveal that calculated doses by NPS and Brachtherapy Planning are higher than measured doses for all distributions studied. Differences between NPS and Brachtherapy Planning were higher for off-axis prescription points. Differences in percent doses for low dose regions were comparable for both fully and posterior loaded plaques.