TH‐C‐224C‐02: MicroRT/microRTP: A Conformal Small Animal Planning and Irradiation System

Purpose: We have developed a novel small animal radiation therapy device (microRT), which integrates multi‐modality imaging, radiation treatment planning, and conformal radiation therapy. In this study, we evaluated the accuracy of the treatment planning and positioning systems of the microRT device. Method and Materials: The microRT system utilizes a clinical ¹⁹²Ir HDR source collimated via machined tungsten inserts to deliver photon beams at a source to target distances of 1–8cm at four angles (0, 90, 180, and 270). Beams were modeled using Monte Carlo and a parameterized analytic dose engine was created. Radiochromic film (5mm steps) in a solid water phantom was used to evaluate actual delivered doses in multiple planes. Treatment plans using these beams were created by a custom treatment planning system (microRTP) based on imported fiducial‐registered imaging (CT, MR, PET) of animals immobilized in the treatment position. A three‐axis computer‐controlled stage supports and positions animals in the beams according to the microRTP plan. Validation of the positioning system was performed using a phantom and images of phantom and collimator via a kV C‐arm. Results: The analytic dose model agreed with the Monte‐Carlo predicted dose within 5% and 10% outside and inside the 1 mm deep build‐up regions, respectively. Film dosimetry agreed with the analytic model within 10% and also demonstrated an effective field diameter of 8mm at 17mm from the source. The ¹⁹²Ir line source geometry caused a radial anisotropy of up to 12% at 17 mm depth from the source. The positioning accuracy of the animal support hardware was sub‐millimeter. Conclusions: The microRT system provides conformal radiation therapy based on pre‐treatment imaging and planning for small animal models of cancer and tissue injury. This work supported in part by NIH R21 CA108677 and by a grant from Varian, Inc.