Quantitative calibration of Tb-161 SPECT/CT in view of personalised dosimetry assessment studies.

Background: Terbium-161 (¹⁶¹Tb)-based radionuclide therapy poses an alternative to current Lutetium-177 (¹⁷⁷Lu) approaches with the additional benefit of secondary Auger and conversion electron emissions capable of delivering high doses of localised damage to micro-metastases including single cells. Quantitative single-photon emission computed tomography, paired with computed tomography (SPECT/CT), enables quantitative measurement from post-therapy imaging. In view of dosimetry extrapolations, a Tb-161 sensitivity SPECT/CT camera calibration was performed using a method previously validated for ¹⁷⁷Lu.

Methods: Serial imaging of a NEMA/IEC body phantom with Tb-161 was performed on SPECT/CT with low-energy high-resolution collimators employing a photopeak of 75 keV with a 20% width. Quantitative stability and recovery coefficients were investigated over a sequence of 19 scans with buffered ¹⁶¹Tb solution at total phantom activity ranging from 70 to 4990 MBq.

Results: Sphere recovery coefficients were 0.60 ± 0.05, 0.52 ± 0.07, 0.45 ± 0.07, 0.39 ± 0.07, 0.28 ± 0.08, and 0.20 ± 0.08 for spheres 37, 28, 22, 17, 13, and 10mm, respectively, when considered across all activity and scan durations with dual-energy window scatter correction. Whole-field reconstructed sensitivity was calculated as 1.42E-5 counts per decay. Qualitatively, images exhibited no visual artefacts and were comparable to ¹⁷⁷Lu SPECT/CT.

Conclusions: Quantitative SPECT/CT of ¹⁶¹Tb is feasible over a range of activities enabling dosimetry analogous to ¹⁷⁷Lu whilst also producing suitable imaging for clinical review. This has been incorporated into a prospective trial of ¹⁶¹Tb-PSMA for men with metastatic prostate cancer.