Progress toward a microradiation therapy small animal conformal irradiator.

Microradiation therapy (microRT) systems are being designed to provide conformal radiation therapy to small animals enabling quantitative radiation response evaluation. We used a Monte Carlo approach to estimate the radiation dose distributions from proposed blueprints and developed a beam model to aid in the microRT system design process. This process was applied to a prototype irradiator that uses a small (3 mm long and 3 mm in diameter), cylindrical, high-activity 192Ir source delivering the radiation beam using custom-fabricated tungsten collimators. The BEAMnrc Monte Carlo code was used to simulate dose distributions from these prototype collimators. Simulations were performed at three source-to-surface distances (50, 60, and 70 mm), and with five circular field sizes (5, 7.5, 10, 12.5, and 15 mm). A dose to a 50 X 50 X 50 mm3 water phantom with 1 X 1 X 1 mm3 voxel spacing was computed. A multiparameter dose calculation algorithm was developed to efficiently and accurately calculate doses for treatment planning exercises. The parametrization was selected so that the parameters varied smoothly as a function of depth, source-to-surface distance, and field size, allowing interpolation for geometries that were not simulated using the Monte Carlo simulation. Direct comparison of the model with the Monte Carlo simulations showed that the variations were within 5% error for field sizes larger than 10 mm, and up to 10% for smaller field sizes.