Development of a novel tool for high-precision focal irradiation using a clinical brachytherapy system

Purpose: This study aims to emphasize the necessity of a focal irradiation tool for small animals and compare the beam characteristics of a tool developed using a brachytherapy system with a linear accelerator (LINAC)-based tool. Methods: A 1-mm tungsten collimator was designed for a Ir-192 brachytherapy system. The percent depth dose (PDD) and horizontal profile of the collimator were measured and compared with a 4-mm commercial cone in the LINAC. Monte Carlo simulations validated all the measurements. Mouse brains were irradiated using a focal irradiation tool, and immunohistochemistry was performed on the brain samples to assess the dose accuracy. Results: PDD showed that the maximum dose (dmax) for Ir-192 was at the surface in both measurements and simulations. At a depth of 1 mm, the collimator measured doses of 25.6 % and 21.0 %, respectively. At 6 MV in the LINAC, the dmax was observed at depths of 0.7 and 0.8 cm in measurements and simulations, respectively. The full width at half maximum (FWHM) at a depth of 1 mm was 1.0 and 1.1 mm for Ir-192 in the measurements and simulations, respectively. For small cone sizes at dmax, FWHM was 4.0 and 4.1 mm for the measurements and simulations, respectively. Immunohistochemistry results indicated that focal irradiation with Ir-192 affected small superficial brain areas while sparing the contralateral side and subventricular zone. Conclusion: The focal irradiation tool accurately delivered doses to small regions and shallow depths in the mouse brain, making it valuable for precise radiotherapy during small animal experiments.