MO‐D‐T‐6E‐09: Progress Towards a MicroRT Small Animal Conformal Irradiator

Purpose: Newly developed small animal imaging devices, like micro computed tomography (microCT), micro positron emission tomography (microPET), and micro magnetic resonance (microMR), have stimulated development of small radiation therapy devices — microRTs. A conformal small animal irradiator will provide customized dose distributions that enable the investigator to limit confounding side effects and obtain more quantitative response results. Method and Materials: The first step towards designing the microRT irradiator was to perform Monte Carlo simulations to aid in optimization of the proposed design. The proposed irradiator uses a high activity [formula omitted] source that is a relatively small (3mm long and 3mm in diameter) cylinder. The BEAMnrc Monte‐Carlo code for was utilized to model the dose distribution for three source‐to‐target distances: 60mm, 70mm and 80mm, and five circular field sizes: 5mm, 7.5mm, 10mm, 12.5mm and 15mm. Finally, dose to a 50 × 50 × 50mm³ water phantom with 1 × 1 × 1mm³ voxel spacing was computed. To provide rapid dose calculations for treatment planning, a parametric dose model was developed and fit to the Monte Carlo data. Results: The simulated radiation beams were determined to be radially symmetric, so a radially symmetric parametric form was selected for the dose model. The depth dose distribution was dominated by the inverse square law and the beam profile and depth‐dose fits were excellent. The parameters varied smoothly as a function of depth, source‐to‐surface distance, and field size, allowing interpolation for non‐simulated geometries. Conclusion: Preliminary results of Monte‐Carlo simulations demonstrated that the parametric fit to the dose distribution of a [formula omitted] microRT device provides good agreement with Monte Carlo predictions.