Applicability of the defined solid angle method to the activity measurement of α-emitters for targeted radionuclide therapy.

We investigated the applicability of the defined solid angle (DSA) approach as a basis for activity measurement of therapeutic α-emitters, focusing on 223Ra and 225Ac. Diaphragms with different aperture diameters were fabricated using a 3D printer and employed to delimit the sensitive area. The heterogeneity of the source activity distribution was evaluated using an imaging plate, and Monte Carlo simulations were employed to calculate the effective solid angle, explicitly accounting for the non-uniform distribution of activity on the source surface. Although the mechanical precision of the present setup was limited, resulting in larger uncertainties than those of conventional DSA methods, our results demonstrate that the geometrical and spectral components required for applying the defined-solid-angle approach to medical α-emitter activity measurements can be evaluated, although a final activity value was not determined in the present study. This study represents a first step toward establishing a practical absolute measurement method for therapeutic α-emitters, with potential for improved accuracy through enhanced mechanical design and metrological control.