Averaging of absorbed doses: How matter matters

Area of Science:

• Medical Physics
• Nuclear Medicine
• Radiotherapy

Background:

• Radionuclide therapy dosimetry frequently necessitates calculating average absorbed doses within and between spatial regions.
• Voxel-based dosimetry, paired organs, and multiple tumors require robust averaging methods for accurate dose assessment.
• Existing methods for averaging absorbed doses vary, impacting the reliability of dose calculations.

Purpose of the Study:

• To formally define and compare various strategies for averaging absorbed doses in radionuclide therapy.
• To evaluate the impact of different averaging methods on non-uniform absorbed dose distributions.
• To ensure consistency and clarity in reporting absorbed dose calculations.

Main Methods:

• Investigated two intra-region averaging methods: simple region average and density-weighted region average.
• Examined three inter-region averaging methods: volume-weighted, mass-weighted, and unweighted averages.
• Applied these methods to schematic, voxel-based, and patient-based (177Lu-PSMA, 177Lu-DOTA-TATE) dosimetry examples, utilizing dose-volume histograms (DVHs) and dose-mass histograms (DMHs).

Main Results:

• Averaging strategies significantly impact absorbed dose estimates, especially with differing region masses and doses.
• Density-weighted region average proved stable and consistent with S-values, unlike the simple region average.
• Inter-region averages (mass-weighted, volume-weighted) were similar, but differed notably from unweighted averages; patient data showed significant deviations with non-uniform densities.

Conclusions:

• The choice of absorbed dose averaging strategy critically influences dosimetry outcomes in radionuclide therapy.
• Explicitly specifying the averaging method employed is essential for accurate and reproducible dosimetry reporting.
• Standardization of averaging techniques will enhance the reliability of radionuclide therapy efficacy and safety assessments.