Alphavision: 12 points go to… Astatine-211

Background and purpose
Alpha-emitting radionuclides and targeted alpha therapy (TAT) are transforming therapeutic and theragnostic nuclear medicine due to their high local dose delivery and possible minimal impact on surrounding healthy tissue. However, their clinical implementation faces challenges, due to complex decay schemes and suboptimal imaging results. Among these, At-211, with its relatively straightforward decay scheme and interesting emission spectrum, has re-emerged as a promising candidate for TAT. This study aims to evaluate the quantitative imaging capabilities of At-211 using a clinical SPECT/CT system.

Materials and methods
Based on previous spectrum measurements [1], the energy windows were based on the At-211’s characteristic X-ray emission from the EC decay to Po-211 (79keV±15%), with an adjacent scatter window (10%) and a supplemental window (245keV±10%) to evaluate the presence of the co-produced contaminant At-210. Imaging was performed on a Siemens Symbia Intevo Bold SPECT/CT system with a LEHR collimator, 128x128 matrix, non-circular step-and-shoot mode with 60 frames of 40s. A large volume (41.83 kBq/mL) was used to determine a calibration factor, while a NEMA phantom was used for contrast recovery analysis.

Results
Multiple NEMA phantom acquisitions demonstrated excellent visualisation of spheres, simulating activity uptake (38.05, 23.80, 1.51 kBq/mL at the time of acquisition). The background was filled with water due to waste concerns. While the smallest sphere in the second acquisition was still clearly visible (12kBq), an activity of 8kBq in the third largest sphere (5.5mL) seemed to be the limit of visualisation. The data was reconstructed using 8ix8s, 8ix10s, 16ix8s, 16ix10, 24ix8s, 24ix10s, 32ix8s, 32ix10s, 64ix8s, 64ix10s, with a gaussian filter of 9mm FWHM. The standard deviation of the mean counts for the 37, 28, 22, 17, 13 and 11mm were 0.04, 0.05, 0.03, 0.05, 0.22 and 0.24%, indicating early conversion of the reconstruction. Further quantification yielded recovery coefficients (smallest to largest spheres) of [0.29, 0.26, 0.37, 0.45, 0.58, 0.79] for the first time point and [0.18, 0.26, 0.32, 0.44, 0.54, 0.78] for the second time point, indicating good correspondence between the acquisitions.

Conclusion
This study confirms the feasibility of quantitative imaging of At-211 using a clinical SPECT/CT system, achieving a visualisation threshold of ±10kBq for small volumes. The results demonstrate reliable reconstruction and activity recovery, supporting the potential integration of At-211 into theragnostic workflows.