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

Postoperative β-irradiation after pterygium excision has been considered a valuable therapeutic procedure to reduce the recurrence rate. Recently, it was reported that β-irradiation also substantially reduced the risk of surgical failure after glaucoma surgery. Pure β-irradiation using a Sr90 Y applicator has been almost exclusively used for this purpose. As an alternative to Sr90 Y β-irradiation, we propose treatment with betas of a P32 source. While P32 has a lower maximum energy (1.71 MeV) than Sr90 Y (2.27 MeV), it has an average energy comparable to that of Sr90 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 P32 applicators were performed using the MCNP5 code. The structure and dimension of the P32 applicators were based on those of the Sr90 Y applicators currently available, while medical plastic encapsulation and liquid source were chosen to enhance β-dose to the surface of the conjunctiva. The P32 applicator showed that the surface dose distribution (up to 0.75 mm depth) is very similar to that of Sr90 Y. However, beyond 0.75 mm depth, the P32 doses decrease with depths more rapidly than Sr90 Y doses. In order to achieve the same surface dose rate, the required P32 activity is about three times that for a Sr90 Y applicator. We conclude that the proposed P32 applicator can deliver therapeutic doses to the target lesion while sparing the lens better than the Sr90 Y applicator. The P32 activity required to deliver therapeutic doses can be produced in a 30 MW reactor available at the Korea Atomic Energy Research Institute.