PHSOR07 Presentation Time: 1:00 PM: Feasibility of Using Dual-Activity Seed Loading When Using Eye Physics Eye Plaques and Plaque Simulator Treatment Planning in Eye Plaque Brachytherapy

Abstract

<h3>Purpose</h3> Uveal melanoma is a tumor of the eye commonly treated with the application of gold plaques loaded with radioactive I-125 seeds. This study evaluates the dosimetric impact of incorporating dual-activity seed loading in Eye Physics eye plaques under two clinical scenarios that may benefit from this dual-activity approach. <h3>Materials and Methods</h3> Plaque Simulator (Eye Physics, LLC, Los Alamitos, CA) was used to retrospectively re-plan 10 clinical cases for two clinical scenarios: 1) shallow (<5.5 mm) tumors with large base dimensions (16 mm - 19 mm) planned with the largest plaque available from Eye Physics (EP2342 in Fig.1), and 2) tumors very near the optic nerve planned with EP2029N notched plaque (Fig. 1). Five clinical plans were selected for each clinical scenario. The clinical plans chosen were prescribed to at least 2 mm deeper depth than the tumor apex in order to achieve target (base + 2 mm margin) coverage of at least 95%. In the replanning, some of seeds in the plaque were replaced with higher activity seeds. For Scenario 1), EP2342 plaques were planned with the outer ring of sources at twice the activity of the inner sources. For Scenario 2), EP2029N plaques were planned with the 6 seeds closest to the notch at twice the activity of the other sources. Seed loading was kept symmetric for ease of loading, QA, and plaque placement. Plans were based on retinal sketches rather than digitized directly from retinal imaging, so the exact dose to critical structures is an estimate, but the direction and approximate magnitude of the dose change can be analyzed. <h3>Results</h3> In all the cases of large shallow tumors for Scenario 1), the dual-activity plan was able to decrease the prescription depth by 0.8 mm to 1.3 mm. Therefore, doses to all critical structures were consistently lower in dual-activity plans: inner sclera -24.6% ± 1.6%, optic disc -6.5% ± 2.5%, fovea -6.0% ± 3.3% (average ± standard deviation). In 3 out of 5 tumors close to the optic nerve for Scenario 2), the dual-activity plan was able to decrease the prescription depth by 0.5 mm to 0.7 mm. In all 5 of these cases, dose to inner sclera was consistently lower in the re-plans by 20.9%±5.6% and dose to the optic disc and fovea were consistently higher by 19.3%±7% and 21.8%±11.4% respectively. <h3>Conclusions</h3> For large shallow tumors, critical structure dosimetrics were constantly improved due to shallower prescription depth by partially loading with higher activity seeds. The largest dose reduction occurs to the inner sclera since it is close to the plaque, but changes in other critical structures were patient-specific depending the location of the tumor in the eye. It may be beneficial to always plan tumors such as these with dual activities. As this is the largest plaque offered by Eye Physics, this planning approach may allow it to be used for larger tumors while still maintaining reasonable dose to critical structures. For tumors near the optic nerve, the results showed that although scleral dose was decreased, dose to optic disc and fovea were increased. As this difference is due to the location of the tumor relative to the optic nerve and the fovea, it may be beneficial to investigate dual activity plans on a case-by-case basis if scleral dose is a concern for that particular patient.