SU‐E‐T‐652: Quantification of Dosimetric Uncertainty of I‐125 COMS Eye Plaque

Purpose: To compare dosimetrically three plan calculation systems (Plato, Varian Brachytherapy, and in‐house‐made Excel) available for I‐125 COMS eye plaque treatment with measurement. Methods: All systems assume homogeneous media and calculations are based on a three‐dimensional Cartesian coordinates, Plato and Brachytherapy Planning are based on AAPM TG‐43 and the in‐house Excel program only on inverse square corrections. Doses at specific depths were measured with EBT3 Gafchromic film from a fully loaded and a partially loaded 16 mm plaque (13 and 8 seeds respectively, I‐125, model 6711 GE, Oncura). Measurements took place in a water tank, utilizing solid water blocks and a 3D‐printed plaque holder. Taking advantage that gafchromic film has low energy dependence, a dose step wedge was delivered with 6 MV photon beam from a Varian 2100 EX linac for calibration. The gray‐scale to dose in cGy was obtained with an Epson Expression 10000 XL scanner in the green channel. Treatment plans were generated for doses of 2200 cGy to a depth of 7 mm, and measurements were taken on a sagittal plane. Results: The calculated dose at the prescription point was 2242, 2344, and 2211 cGy with Excel, Brachyvision and Plato respectively for a fully loaded plaque, for the partially loaded plaque the doses were 2266, 2477, and 2193 cGy respectively. At 5 mm depth the doses for Brachyvision and Plato were comparable (3399 and 3267 cGy respectively), however, the measured dose in film was 3180 cGy which was lower by as much as 6.4% in the fully loaded plaque and 7.6% in the partially loaded plaque. Conclusion: Careful methodology and calibration are essential when measuring doses at specific depth due to the sensitivity and rapid dose fall off of I‐125.