A study of differences in absorbed dose estimates by images used in dosimetry of Lu-177-DOTATATE therapy

In Lu-177-labeled peptide receptor radionuclide therapy, dosimetry has recently gained importance for assessing side effects and treatment responses. However, no standard method has been established yet. In this retrospective study, we compared the kidney-absorbed doses of 24 treatments with Lu-177-DOTATATE using three methods: a planar method using only planar images, a hybrid method using planar and SPECT/CT images, and a SPECT/CT method using only SPECT/CT images. In the Planar method, the ROI was defined from 2D whole-body planar images and calculated using the MIRD method. In the Hybrid method, the VOI was defined from CT images and the VOIs were placed in the 2D planar image as ROIs, which were calculated using the MIRD method. For the SPECT/CT method, the VOI was defined from CT images and the mean absorbed dose was estimated on a voxel basis. The absorbed dose estimated with the Planar method (15.2 ± 5.81 Gy) was significantly higher than the estimates with the other two methods (Hybrid: 2.93 ± 1.33 Gy, SPECT/CT: 3.81 ± 0.93 Gy) (p < 0.05). The Hybrid and SPECT/CT methods exhibited the strongest correlation. The Planar method demonstrated the highest variability in estimated values. The use of 2D planar images alone tended to overestimate the absorbed dose compared to the other methods, depending on the definition of the ROIs and the characteristics of the analysis software. This suggests that a combined approach using SPECT/CT and planar images is preferable for dosimetry.