Feasibility of total body dosimetry of Lu-177 SPECT-CT images recorded by a CZT camera

1590 Introduction: 177Lu radionuclide therapy remains difficult to monitor with current gamma-camera systems due to the low-count rate conditions of 177Lu imaging. Recently, major technical progress has been achieved for CZT-SPECT imaging systems. The Veriton® camera (Spectrum Dynamics Medical) is a total-body 360° CZT-camera equipped with 12 swiveling detectors, which can be positioned close to the patients during the entire acquisition, thereby providing a high count-sensitivity that is particularly advantageous in 177Lu imaging. The aim of this study was to evaluate the feasibility of 177Lu SPECT dosimetry based on < 30 min total-body recordings with this high sensitivity CZT camera. Methods: In a first step, the accuracy of absolute quantification has been validated on SPECT images using an IEC body phantom containing 6 spheres filled-in with 8:1 ratio of 177Lu. In a second step, SPECT and CT images were obtained from 3 patients with neuroendocrine tumors, who received a full-dose Lutathera® injection. Total-body acquisitions, involving 6 consecutive bed positions, each of 2 to 4 mins recording-time, were obtained at 4, 24, 48 and 168 hours after the injection. All SPECT images were obtained from the 113 keV (±10%) energy peak and reconstructed with corrections for attenuation and scatter, resolution recovery and partial volume correction. Mean doses for kidneys, liver and spleen, as well as mean doses for lesions and the total-body, were calculated using MIM SurePlan MRT software. Results: Signal-to-noise ratios of the total-body SPECT images were very high, even for those recorded 7 days after Lutathera® injection. On average, the absorbed doses were measured at 0.27±0.05 mGy/MBq for kidneys, 0.40±0.34 mGy/MBq for liver and 0.34±0.16 mGy/MBq for spleen, and these values are close to those commonly documented on the second energy peak of 177Lu (208 keV) acquired by conventional gamma-cameras equipped with medium energy collimators. The total-body dosimetry could additionally be assessed at a level of: 0.032±0.005 mGy/MBq. Conclusions: This study demonstrated that this high-sensitivity 360° CZT-camera may provide 177Lu therapy dosimetry on a total-body scale for use in clinical routine, with acceptably fast recording times and based on the lower 113 keV energy peak.