P216] Dosimetric study of several beta-emitting ophthalmic plaques with different plaque materials, by Monte Carlo simulation

Abstract

Purpose Dosimetric investigation of brachytherapy eye plaques (based on CCB, COB and CIB models) for different coated radionuclides and different plaque materials for the treatment of uveal melanoma. Methods An eye model (including cornea, lens, optic nerve and tumor) and a plaque (with 0.1 mm window, 0.2 mm radionuclide coated foil and 0.7 mm backing), were simulated in a spherical water phantom, using Geant4.10.4. Dose-depth profiles were plotted for Ru-106, Sr-90, Ho-166, Re-188 and Pr-142 plaques. The dose deposited in the tumor and behind the plaque’s backing was calculated for the silver, gold and stainless steel plaques. For validation, dosimetry of CCB, COB and CIB plaques were performed using radiochromic EBT3 film and Plaque Simulator software, and the obtained dose rates were compared with Geant4.10.4 and the manufacturer’s (BEBIG) results. Results The ratio of dose at a point behind the silver backing to the reference point (2 mm depth) was 1.56%, 0.13%, 0.28%, 0.39% and 0.18% for Ru-106, Sr-90, Ho-166, Re-188 and Pr-142 CCB model plaques, respectively. The dose rate at the reference point was 66.4%, 50.7%, 34.8%, 41.87% and 47.1% of the dose rate at a depth of 0.5 mm for the mentioned plaques, respectively. With the same activity, Ho-166 plaque delivered the highest dose to the scleral, while Ru-106 plaque delivered the lowest. The dose per decay at the tumor apex (5 mm depth) was 3.59e−11 Gy, 5.27e−12 Gy, 7.96e−13 Gy, 2.24e−12 Gy and 3.36e-12 Gy for the mentioned plaques, respectively. The deposited dose in the tumor decreased by 50% for gold Ru-106 plaques and increased by 20% for stainless steel ones. Conclusions From the perspective of dose distribution, Ru-106 plaques can be replaced by Sr-90 or Pr-142 plaques for the tumor apex up to 3 mm. The dose fall-off for Ho-166 and Re-188 plaques are more severe, which limits their use only for superficial lesions. Gold plaques show more attenuation and can be made thinner. They may be used in COB and CIB models, due to their vicinity of optic nerve and iris, respectively. For tumor apex up to 5 mm only Ru-106 delivered the sufficient dose.