SU-E-T-396: Heterogeneity Effect on Dose Distributions for COMS Eye Plaques with 125I Seeds (Model I25.S16): A Radiochromic EBT Film Dosimetry Study

Abstract

Purpose: To study the heterogeneity effect on dose distributions in eye phantom for COMS eye plaques with 125I seeds using radiochromic EBT film dosimetry. Methods: COMS eye plaques (14,16,18 and 20mm in size) uniformly loaded with 125I seeds (model I25.S16) were studied. EBT film (lot #36076‐003AL) was positioned in a polystyrene eye phantom above the eye plaque, in two configurations: (1) in the plaque's central plane, (2) perpendicular to the central axis at depth=5 or 12mm (from inner sclera) in the eye phantom. With the seed air‐kerma‐strengths of 2.4–4U, the exposure times (3.5–8.7h) were adjusted to deliver ∼400cGy at 5mm depth. Calibration films were irradiated by a single 125I seed at ∼5mm distance, one at a time, for doses up to 1200cGy. All films were scanned using Epson 10000XL scanner and data were analyzed using Mephysto FilmCal Mc2 software. With the established calibration curve, dose conversion was performed. Central axis depth doses and off‐axis profiles were determined. Results: The film dose data were compared with the calculated doses using Plaque Simulator v5.3.9 with homogeneous assumption (Homo) and heterogeneity correction (Hetero). The dose ratio (film/Homo) and (film/Hetero) values were obtained. The dose ratio (film/Homo) values are substantially lower than unity (mostly between 0.8 and 0.9) for all plaque sizes studied, indicating dose reduction by COMS plaque compared with homogeneous assumption. The film dose data agree with the Hetero values within the uncertainty of measurement data. Conclusions: We found significant heterogeneity effect on the 125I (model I25.S16) dose distributions in an eye for COMS plaques using radiochromic EBT film dosimetry. The film data agree with the calculated doses using Plaque Simulator with heterogeneity correction. The dose reduction effect for COMS plaques uniformly loaded with 125I seed (model I25.S16) found in this study is similar to that for other 125I seed models.