Normal-Organ Effective Half-Lives for 177Lu-PSMA
Abstract
Purpose
To report effective half-lives of normal organs and investigate the relationship between kidney effective half-life and creatinine clearance in patients undergoing 177Lu-PSMA-617 therapy at our institution. As single-timepoint dosimetry relies on patient-kinetics matching well with population kinetics, this study addresses the current scarcity of reported effective half-life values.
Methods
Twenty patients undergoing 177Lu-PSMA-617 therapy with post-treatment dosimetry were analyzed, each imaged with SPECT/CT at 4-, 24-, 48-, and 72-hours post-administration. Reasons for dosimetry included: re-treatment (n= 1), solitary kidney (n=3), lung disease (n=4), renal disease (n=2). Regions analyzed were the kidneys (n=37), lungs (n=40), liver (n=20), spleen (n=20), parotid gland (n=38), and bone marrow (n=20). Organs were contoured in MIM Software (MIM Software, Cleveland, OH) using MIM Contour ProtégéAI, and a representative bone marrow region was contoured within lumbar vertebrae. Effective half-lives were estimated by fitting a monoexponential model to activity concentration data using the SciPy (v1.17.0) curve-fitting function in Python. Creatinine clearance was estimated from serum creatinine using the Cockcroft-Gault formula. For each region, descriptive statistics (mean, median, IQR, range) were calculated. Spearman’s rank correlation was used to examine the relationship between kidney effective half-life and creatinine clearance.
Results
Median effective half-lives (range) were: kidneys 35.5 h (8.5-152.0), lungs 15.1 h (7.4-160.8), liver 22.9 h (12.9-63.4), spleen 16.8 h (9.3-149.7), parotid gland 42.2 h (18.9-160.8), and bone marrow 32.0 h (5.9-160.8). No correlation was found between kidney half-life and creatinine clearance (p=0.278).
Conclusion
Effective half-lives for kidney and liver were comparable to those reported in literature. However, the parotid gland effective half-life was higher than reported values, and published values could not be found for image-based red marrow, lungs, or spleen. Outliers were present in each organ, which warrants future investigation to identify characteristics of patients with normal clearance to qualify candidates for single-timepoint dosimetry.