Modelling the relative precision of whole-kidney dosimetry in molecular radiotherapy using a power law approach

A power-law model was introduced to characterize the relationship between spherical volumes-of-interest (SVs) and whole kidney parenchyma (WKP)-derived absorbed dose estimates, enabling quantitative precision for image-based dosimetry. Single photon emission computed tomography/computed tomography (SPECT/CT) images were acquired at 24, 48, and 168 h after [177Lu]Lu-DOTATATE treatment in 18 patients. Kidney activity was quantified using WKP and SV-based methods (2 and 0.6 ml) on SPECT-images. WKP and both SV approaches showed good agreement in kidney dosimetry, with normalization factors of 1.12 and 1.23 (standard error mean ≤1.2%), and improved precision when multiple SVs were used. The power-law model demonstrated excellent fit (R2 > 0.97) and high precision (~7%), with no significant difference between SV sizes (P = .15) and minimal bias (<0.003%). This power law model presents a novel method for quantifying the relative precision of SPECT-derived kidney dosimetry. Further validation is warranted to address residual uncertainty and complex noise correlation in the SV-based dose estimates.