Quantitative ¹⁷⁷Lu-SPECT/CT imaging and validation of a commercial dosimetry software

Full text of publication follows. Aim: 3D dosimetry is an appealing yet complex application of SPECT/CT in patients undergoing radionuclide therapy. In this study we have developed a quantitative imaging protocol and we have validated commercially available dosimetry software (Dosimetry Tool-kit Package, GE Heathcare) in patients undergoing ¹⁷⁷Lu-DOTATATE therapy. Materials and methods: dosimetry tool-kit uses multi SPECT/CT and/or WB planar datasets for quantifying changes in radiopharmaceutical uptake over time to determine residence times. This software includes tools for performing reconstruction of SPECT/CT data, registration of all scans to a common reference, segmentation of the different organs, creating time activity curves, curve fitting and calculation of residence times. All acquisitions were performed using a hybrid dual-head SPECT-CT camera (Discovery 670, GE Heathcare) equipped with medium energy collimator using a triple-energy window. SPECT images were reconstructed using an iterative reconstruction algorithm with attenuation, scatter and collimator depth-dependent three-dimensional resolution recovery correction. Camera sensitivity and dead time were evaluated. Accuracy of activity quantification was performed on a large homogeneous source with addition of attenuating/scattering medium. A NEMA/IEC body phantom was utilized to measure the recovery coefficient that the software does not take into account. The residence times for organs at risk were calculated in five patients. OLINDA-EXM software was used to calculate absorbed doses. Results: ¹⁷⁷Lu-sensitivity factor was 13 counts/MBq/s. Dead time was <3% with 1.11 GBq in the field of view. The measured activity was consistent with the decay-corrected calibrated activity for large volumes (>100 cc). The recovery coefficient varied from 0.71 (26.5 ml) to 0.16 (2.5 ml) in the absence of background activity and from 0.58 to 0.13 with a source to background activity concentration ratio 20:1. The activity concentration measured in the spheres depended on the reconstruction parameter and for the smallest sphere the accuracy increased with increasing numbers of iterations and subsets. The residence time range was 0.33-1.46 hours for lungs, 0.57-3.54 hours for spleen, 0.93-3.58 hours for kidneys and 3.74-8.33 hours for liver. The mean absorbed dose was 0.69 Gy/GBq for liver, 0.76 Gy/GBq for spleen, 0.55 Gy/GBq for kidneys and 0.08 Gy/GBq for lungs. Conclusion: quantitative SPECT with Lu¹⁷⁷ has high accuracy both in our phantom and in clinical practice. Dosimetry tool-kit simplifies the procedure for quantifying absorbed dose reduces the processing time and improves accuracy of results compared to manual Methods. Software implementation of partial volume correction in the dosimetric work-flow is recommended to obtain more accurate dose estimations. (authors)