Imaging Alpha's Or: How I Learned to Stop Worrying and Love At-211

Background
Alpha-emitting radionuclides have gained significant attention in targeted alpha therapy due to their high linear energy transfer and short range, offering promising therapeutic potential with minimal damage to surrounding healthy tissues. Among these, At-211 has emerged as a candidate with favourable properties. This study aims to evaluate the imaging capabilities of At-211 using a clinical SPECT/CT system.
Methods
A point source and a flask containing a known concentration of 41.83 kBq/mL of At-211 were imaged using a Siemens Symbia Intevo Bold SPECT/CT system to characterize the emission spectrum with and without collimators (LEHR) mounted on the detector heads. Subsequently, the spheres of a NEMA phantom were filled with 39.73 kBq/mL of At-211 to simulate lesion uptake.
Results
Based on Monte Carlo simulations and confirmed by the measured spectrum, the energy windows were based on the characteristic X-ray production due to the At-211 EC Decay and were defined as 79±12keV for the main emission and 59±8keV for lower scatter. A supplemental window was placed at 245±24.5keV to evaluate the presence of co-produced contaminant At-210. Imaging was performed using a 128x128 matrix, LEHR collimator in non-circular step-and-shoot mode with 60 frames of 40s. The imaging experiments yielded excellent visualisations of the activity in all phantoms, even showing activity in the smallest sphere (10mm diameter) of the NEMA phantom, affirming its potential for targeted alpha therapy. Quantification was possible, as a calibration factor of 34.38±0.51cps/MBq was calculated using the large volume of the bottle. Quantification uncertainty for NEMA spheres is larger [3-6 cps/MBq] for the three largest spheres.

Conclusion
Our study demonstrates the successful imaging and quantification of At-211 using a clinical SPECT/CT system. Further optimisations using Monte Carlo simulations are being performed to determine the contamination of the acquired At-211 spectrum as a consequence of photon interactions with the equipment (collimator,…) and its influence on image optimization and quantification. These findings support further research and development of At-211-based targeted alpha therapy for clinical applications and further imaging optimisations, paving the way for personalized and effective cancer treatment strategies.