TH‐C‐BRC‐04: Small Animal Stereotactic Irradiator

Purpose: We have developed an image‐guided small animal stereotactic irradiator capable of high dose rate radiation delivery and high accuracy in target localization in a manner that mimics clinical stereotactic delivery. In this study, we characterized radiation beams for small animal radiosurgery treatment planning. Method and Materials: The irradiator employs a commercial x‐ray device (XRAD 320, Precision X‐Ray, Inc.) with custom made collimation system consisting of a brass collimator holder and variable tungsten alloy collimators with apertures ranging from 1 to 10 mm in diameter. The image guidance system consists of a high resolution x‐ray intensifying screen (Kodak Min‐R2) coupled to a computer‐controlled digital camera. The x‐ray tube is operated at 30 kVp for imaging, and at 250 kVp for therapeutic delivery. To characterize the radiation beams, we measured percent depth dose curves (PDD), off‐axis ratios (OAR) and absolute dose rates for each collimator using radiochromic film (Gafchromic EBT, International Specialty Products, Wayne, NJ) at 19.76 and 24.76 cm source‐to‐surface distance (SSD). Results: For a 5 mm circular collimator, at 19.76 cm SSD, the penumbra, defined as distance between 80% and 20% isodoses, for in‐plane and cross‐plane measurements was 0.7 and 1.1 mm, respectively. For the 5mm collimator, PDD values of 55% and 20% were at depths of 25 and 50 mm respectively, indicating more than adequate penetration for deep seated targets in small animals. Absolute dose rates ranged from 6.7 to 11.6 Gy/min for the 1 to 10 mm collimators at SSD of 19.76 cm. At 24.76 cm SSD, for 5 and 10 mm collimators, measured dose rates were 7.1 and 7.7 Gy/min, respectively. Conclusion: The irradiator offers high dose rates and sharp beam profiles, and has been systematically characterized for a set of source‐to‐surface distances and field sizes relevant for a number of animal experiments already successfully performed.