Dual-source strength seed loading for eye plaque brachytherapy using eye physics eye plaques: A feasibility study

Purpose
This study quantifies the dosimetric impact of incorporating two iodine-125 (¹²⁵I) seed source strengths in Eye Physics eye plaques for treatment of uveal melanoma.

Material and methods
Plaque Simulator was used to retrospectively plan 15 clinical cases of three types: (1) Shallow tumors (< 5.5 mm) with large base dimensions (range, 16-19 mm); (2) Tumors near the optic nerve planned with notched plaques; and (3) Very shallow (< 3.0 mm) tumors with moderate base dimensions (range, 13.5-15.5 mm) planned with larger plaques than requested by the ocular oncologist. Circular plaques were planned with outer ring sources twice the source strength of inner sources, and notched plaques with the six seeds closest to the notch at twice the source strength.

Results
In cases of type (1), the dual-source strength plan decreased prescription depth, and doses to critical structures were lower: inner sclera –25% ±2%, optic disc –7% ±3%, and fovea –6% ±3%. In four out of five cases of type (2), the dual-source strength plan decreased prescription depth, and dose to inner sclera was lower (–22% ±5%), while dose to optic disc (17% ±7%) and fovea (20% ±12%) increased. In cases of type (3), a smaller dual-source strength plaque was used, and scleral dose was lower (–45% ±3%), whereas dose to optic disc (1% ±14%) and fovea (5% ±5%) increased.

Conclusions
Dual-source strength loading as described in this study can be used to cover tumor margins and decrease dose to sclera, and therefore the adjacent retina, but can either decrease or increase radiation dose to optic disc and fovea depending on location and size of the tumor. This technique may allow the use of a smaller plaque, if requested by the ocular oncologist. Clinical determination to use this technique should be performed on an individual basis, and additional QA steps are required. Integrating the use of volumetric imaging may be warranted.