Purpose: Radiopharmaceutical therapies for neuroendocrine tumors have demonstrated promising outcomes, and Lutetium-177-DOTATATE therapy recently received FDA-approval. To implement patient-specific dosimetry for therapy planning and verification, the absorbed dose values should be comparable between patients and centers independent of dosimetry method. Therefore, our aim was to compare 3 different dosimetry calculation methods for healthy organs and tumors.
Methods: The Lutetium-177-DOTATATE patient data sets of the SNMMI Dosimetry Challenge were used for this analysis. At first, a custom voxelized dosimetry approach was assessed, 3DVoxDos. The time-integrated activity (TIA) map was convolved with a Lutetium-177 soft tissue dose kernel (obtained with GATE Monte Carlo simulations) in an in-house MATLAB code. Subsequently, the voxel densities obtained from patient CTs was used to adjust for the density difference to soft tissue. For an organ dosimetry approach, the total TIA and mean density per given volume of interest (VOI) were extracted from TIA maps and CT-derived density maps. Organ dosimetry was performed using IDAC Dose 2.1 and OLINDA 2.2.0 with mass-scaling of the S-factors. Percentage differences (PD) in mean organ and tumor doses between the three different approaches was assessed.
Results: Average PDs of -8.0% (min:-20.1%, max:1.3%), -9.3% (min:-21.3%, max:-0.6%), and 1.4% (min:0.1%, max:2.5%) were found taking all healthy organs together (kidneys, liver, spleen) for 3DVoxDos vs. OLINDA, IDAC vs. OLINDA, and 3DVoxDos vs. IDAC, respectively. For tumors (6 in total), average PDs of 7.5% (min:2.2%, max:11.8%), 7.4% (min:5.1%, max:10.2%), and 0.1% (min:-4.8%, max:4.8%) were found for 3DVoxDos vs. OLINDA, IDAC vs. OLINDA, and 3DVoxDos vs. IDAC, respectively.
Conclusion: Our comparison showed differences between dosimetry calculation methods. IDAC and 3DVoxDos showed overall closest agreement. Largest differences were found for the spleen between IDAC and OLINDA, which could be related to the different S-factor sources. We believe this comparison highlights the importance to standardize radiopharmaceutical therapy dosimetry.