RADIOISOTOPES

Targeted alpha therapy is attracting significant attention for the treatment of intractable tumors and the development of efficient gamma cameras to evaluate the biodistribution of alpha emitters is expected. ²¹¹At, which is a commonly used alpha emitter in Japan, emits characteristic X-rays whose energy is distributed between 77 and 92 keV. We evaluated the biodistribution of this alpha emitter by effectively acquiring these photons. Because the performance of a gamma camera strongly depends on that of collimators, we designed and manufactured high-performance collimators by processing pure tungsten, which can shield radiation more effectively than lead, using state-of-the-art three-dimensional (3D) printing technology. We successfully fabricated a parallel-hole collimator with a honeycomb structure with a hole size of 1.0 mm and septal thickness as thin as 0.07 mm. When a point source of ¹³³Ba that emits photons with the energy of 81 keV, similar to that of characteristic X-rays emitted from ²¹¹At, was imaged using this collimator, spatial resolution of this collimator was as short as 5.7 mm and its efficiency was as good as 1.42×10⁻⁴; these results are superior to those of commercially available clinical scanners. We also demonstrated that our collimator could successfully depict a shape of the object by imaging a mouse thyroid gland after the injection of the ²¹¹At solution. These results suggest that excellent collimators made of pure tungsten could be used to monitor targeted alpha therapy using ²¹¹At.