CALCULATING LENS DOSE AND SURFACE DOSE-RATES FROM SR-90 OPHTHALMIC APPLICATORS USING MONTE-CARLO MODELING

Using a Sr-90 applicator for brachytherapy for the reduction of recurr ence rates after pterygium excisions has been an effective therapeutic procedure. Accurate knowledge of the dose being applied to the affect ed area on the sclera has been lacking, and for decades inaccurate est imates for lens dose have thus been made. Small errors in the assumpti ons which are required to make these estimates lead to dose rates chan ging exponentially because of the attenuation of beta particles. Monte Carlo simulations have been used to evaluate the assumptions that are now being used for the calculation of the surface dose rate and the c orresponding determination of lens dose. For an ideal Sr-90 applicator , results from this study indicate dose rates to the most radiosensiti ve areas of the lens ranging from 8.8 to 15.5 cGy/s. This range is bas ed on different eye dimensions that ultimately corresponds to a range in distance between the applicator surface and the germinative epithel ium of the lens of 2-3 mm. Furthermore, the conventional 200 cGy thres hold for whole lens cataractogenesis is questioned for predicting comp lications from scleral brachytherapy. The dose to the germinative epit helium should be used for studying radiocataractogenesis. (C) 1998 Ame rican Association of Physicists in Medicine.