Dose rates to the human lens from a SR-90 applicator modeled with Monte Carlo computer techniques

Strontium-90 (beta-emitting) applicators for the treatment of pterygia carry the potential for significant (possibly cataractogenic) lens doses previously calculated with imprecise or inaccurate assumptions. We sought to apply modern dosimetry techniques to this problem using the Monte Carlo computer code MCNP version 4.A to model the human eye irradiated with a commonly employed Sr-90 applicator. The code was configured to compute dose rate to any part of the eye, focusing on those tissues (sclera, lens) directly under the applicator. The model was validated by calculating the surface dose delivered by the applicator to the sclera from MCNP results and comparing this to the manufacturer specified value of this parameter. The scleral surface dose rate from a 1.85 GBq (50 mCi) Sr-90 source was 60.5 cGy/sec, with an average dose rate to the underlying sclera about 42 cGy/sec. In the surface region of the lends under the 1.85 GBq Sr-90 applicator, with depth 0.5 m width 1.5 mm over a 90{degrees} arc tangential to the applicator, the dose rate was 6.6 cGy/min, and in the next underlying region of the lens with 0.5 mm thickness 4.3 cGy/min. Isodose curves have been calculated for the lens and entire eye. Our computer programmore » permits calibration of Sr-90 applicators and provides accurate dosimetry from these applicators to the sclera and all regions of the lens. A total lens dose is meaningless. The periphery of the lens will receive cataractogenic doses when a commonly employed dose (20 Gy) for pterygium treatment is used.« less