SU‐GG‐T‐37: Dosimetric Comparison of Cs‐131 to I‐125 for Treatment of Occular Melenoma

Purpose: To determine the conditions where it is advantageous to use [formula omitted] as an alternative to [formula omitted] in the brachytherapy treatment of ocular melanoma. Method and Materials: The dosimetry of nine previously treated patient plans was evaluated comparing [formula omitted] seeds and [formula omitted] seeds in identical configurations of standard eye plaques. Calculations were performed following the TG43 protocol in ADAC Pinnacle Treatment planning software. The resulting doses to prescription point and other structures were compared for the same plan using the two different isotopes. In addition, comparisons using [formula omitted] were performed so that a prescribed dose of 85Gy was achieved, first by adjusting the source strength and second by adjusting the treatment time. [formula omitted] plans were also created for a dose range of 60–95 Gy to determine optimal conditions where [formula omitted] may provide dosimetric advantage compared to [formula omitted]. Results: 12% reduction in prescribed dose to the tumor is observed for the case where the [formula omitted] seed is replaced by a [formula omitted] seed with identical air kerma strengths. Other points of interest show a reduction in dose from 6 to 13 %. An average increase of 14 % for the [formula omitted] source strength results in a dose increase up to 10 % for other points of interest. An average increase of 15 % in treatment time results in a dose increase up to 10 % for some points of interest. Equivalent doses were found for risk structures when 70–75 Gy for [formula omitted] was used. Doses below 75 Gy demonstrated reduced dose to critical structures. Conclusion: For a given prescription dose at the tumor apex, [formula omitted] delivers greater doses to critical structures. Use of [formula omitted] as a source for ocular melanomas may provide a dosimetric advantage for at risk structures if the biological equivalent dose is found to be 75 Gy or less.