SU‐FF‐T‐323: Sub‐Millimter Image Based Radiotherapy Treatment System for Small Animals

Purpose: The sub‐millimeter animal imaging modality is a new field for study molecular or animal images. Such high resolution images can help us study radiation‐induced biological response in small animals. The spatial resolution of current used human treatment system is at least 2 mm, which is too large for the organ dimension of a mouse (5 mm). We have developed an animal RT system based on a Varian Clinac 2100, and a dose calculation kernel using BEAMnrc. Method and Materials: As the result of lateral electron disequilibrium, it is difficult to measure dose characteristic for small beam field. Therefore, our Monte Carlo simulation was verified with a small volume ion chamber and a PTW diamond detector for a larger beam (6MV, Varian Clinac 2100 equipped with a BrainLab 14mm cone). More verification is performed for 4mm, and 6mm cones. A 5‐cm long cylinder with a radius of 1.5 cm is modeled as the head of mouse. We have calculated the dose distribution inside this cylinder for 1‐portal, 4‐portal, and 192 degree arc irradiation. Results: The PDD and profile from measurement and simulation agree to each other for 14mm cone within 0.5% or 0.5 mm. If fitting PDD, we can observe that the curves after D_max decays exponentially. The dose distribution inside the cylinder modeling mouse's head is too sharp for 1‐portal and 4‐portal exposures, which recommends the arc technique as a better approach. Conclusion: We have demonstrated that it is feasible to build a small animal RT system using Varian Clinac 2100 with cones and Monte Carlo dose calculation kernel.