Methodology for comparing absorbed dose rate calculation algorithms in molecular radiotherapy dosimetry

This study compared the results obtained using three of the most frequently proposed algorithms, Local Energy Deposition (LED), Convolution of Voxel S Values (CONV) and Monte Carlo modeling (MC).

OpenDose3D, a free, open-source clinical dosimetry software was used to perform this comparison. The assessment focused on absorbed dose rate (ADR), thereby increasing the dataset size and avoiding potential bias associated with the time integration step. From patients treated with ⁹⁰Y-microspheres, [¹⁷⁷Lu]Lu-DOTATATE or Na[¹³¹I]I, 52 datasets were processed. Voxel-based ADR maps were computed in homogeneous (water) and heterogeneous (CT-derived) media, for a total of 312 datasets. In a heterogeneous medium, density correction was applied at the voxel level. ADR values were averaged over VOIs, and relative differences (rd) were calculated using the MC results as the reference.

In the homogeneous medium, LED underestimated the ADR by up to 10 % in soft tissues, particularly when cross-irradiation cannot be neglected. Conversely, the ADR obtained with CONV presentedexcellent agreement with the MC simulations (rd ≤ 1 %). In the heterogeneous medium, density correction was crucial. For example in ¹³¹I, LED underestimated the ADR values by up to 50 % in lungs and bone marrow. CONV showed excellent agreement with MC in soft tissues (rd ∼ 1 %) and good agreement in organs/tissues where self-irradiation was predominant. The implemented density correction was much less efficient for organs/tissues that experience mostly cross-irradiation.

Our work proposes a procedure for evaluating ADR algorithms. It also underscores the impact of the medium heterogeneity and cross-irradiation on dosimetry calculations.