Potential use of P-32 ophthalmic applicator: Monte Carlo simulations for design and dosimetry

Postoperative beta-irradiation after pterygium excision has been considered a valuable therapeutic procedure to reduce the recurrence rate. Recently, it was reported that beta-irradiation also substantially reduced the risk of surgical failure after glaucoma surgery. Pure beta-irradiation using a 90Sr/Y applicator has been almost exclusively used for this purpose. As an alternative to 90Sr/Y beta-irradiation, we propose treatment with betas of a 32P source. While 32P has a lower maximum energy (1.71 MeV) than 90Sr/Y (2.27 MeV), it has an average energy comparable to that of 90Sr/Y. Furthermore, it can be produced easily in a nuclear reactor by neutron activation and is considered a less hazardous material. Monte Carlo simulations for the dosimetry of proposed 32P applicators were performed using the MCNP5 code. The structure and dimension of the 32P applicators were based on those of the 90Sr/Y applicators currently available, while medical plastic encapsulation and liquid source were chosen to enhance beta-dose to the surface of the conjunctiva. The 32P applicator showed that the surface dose distribution (up to 0.75 mm depth) is very similar to that of 90Sr/Y. However, beyond 0.75 mm depth, the 32P doses decrease with depths more rapidly than 90Sr/Y doses. In order to achieve the same surface dose rate, the required 32P activity is about three times that for a 90Sr/Y applicator. We conclude that the proposed 32P applicator can deliver therapeutic doses to the target lesion while sparing the lens better than the 90Sr/Y applicator. The 32P activity required to deliver therapeutic doses can be produced in a 30 MW reactor available at the Korea Atomic Energy Research Institute.