Optimizing ¹³¹I-mIBG Dosimetry: Validation of Simplified Time-Point and Segmentation Approaches

To evaluate simplified dosimetry methods for ¹³¹I-mIBG therapy that maintain agreement with reference method while reducing clinical workflow burdens, particularly beneficial for pediatric applications.

In 24 patients, we implemented a hybrid protocol combining planar whole-body scans (1, 24, 48 h p.i.) with 24 h SPECT/CT for whole-body and liver dosimetry. Three liver segmentation methods were compared on SPECT/CT: whole-organ manual delineation (reference), 4 mL homogeneous sphere, and 4 mL peak sphere. To estimate time-integrated activity coefficients (TIACs) we modeled biokinetics with monoexponential using: full three-time-point data (reference), reduced combinations (1-2, 1-3, 2-3), Hänscheid single-time-point method, and population-based half-lives. Statistical analysis included linear mixed-effects modeling for segmentation comparisons and Bland-Altman analysis for TIAC method validation.

The median absorbed dose was 0.36 mGy/MBq for liver and 0.09 mGy/MBq for whole-body. Among simplified protocols, dual-time-point 24-48 h imaging demonstrated the least bias, with liver absorbed doses showing - 0.02 mGy/MBq (limits of agreement: -0.08 to 0.04 mGy/MBq) and whole-body absorbed doses 0.001 mGy/MBq (limits of agreement: -0.004 to 0.005 mGy/MBq) compared to the full three-time-point reference results. Among single-time-point methods, for both liver and whole-body, the Hänscheid approach applied at 48 h demonstrated superior performance. The 4 mL peak sphere overestimated absorbed doses by 69% versus whole-organ delineation (0.61 vs. 0.36 mGy/MBq, p < 0.001), and homogeneous spheres underestimated by 8% (0.33 vs. 0.36 mGy/MBq, p = 0.046).

Clinically feasible ¹³¹I-mIBG dosimetry can be achieved through: (1) dual 24 h/48 h imaging (< 6% mean bias), (2) single 48 h Hänscheid method (< 6% mean bias), and (3) homogeneous sphere liver segmentation (< 8% mean difference from reference).