BLUE RIBBON POSTER RADIOPHARMACEUTICALS: Comparing Absorbed Dose Variability In ¹⁷⁷Lu‑PSMA Therapy across Calculation Methods and Commercial Software
Abstract
Purpose
Dosimetry software programs use different computational approaches which may lead to clinically relevant differences in reported absorbed doses (AD). This study aims to evaluate the impact of software methodology on AD calculations.
Methods
This retrospective analysis included omHSPC patients treated with [177Lu]Lu-PSMA-I&T who underwent three-timepoint SPECT/CT imaging (4, 24, 72-96h) post-injection. Patients included had lesions meeting the following criteria for lesions on the 24h timepoint SPECT/CT: tumor-to-background ratio>5, SUVmax>1, and contrast-to-noise>5. Voxel-S-Value and Monte carlo based absorbed dose computations were compared from MIM Software and TORCH by Voximetry respectively. Identical segmentations for the kidneys, liver, salivary glands, and lesions were used in both programs. Segmentations coregistered between all timepoints were manually verified; time activity data for each segmentation was fit using a mono-exponential function; and identical phantom-based partial volume correction factors were applied.
Results
27/46 (59%) patients injected with 7020±208 MBq of [177Lu]Lu-PSMA-I&T were included in the analysis. 40/85 (47%) lesions met the inclusion criteria. 14/40 lesions were bone based while the rest (26/40) were lymph node based. The average AD by MIM Software was 5.94±7.52, and 4.88±6.40 Gy by TORCH by Voximetry respectively. For the lesions, the percent difference between the two programs was 15±26%. The average AD to the kidneys, liver, parotids, and submandibulars by MIM were 2.52±0.75, 0.19±0.10, 1.42±0.71, and 1.65±0.74 Gy, and by TORCH were 2.40±0.70, 0.20±0.11, 1.20±0.66, and 1.19±0.66 Gy, respectively. The percent difference between the reported AD from the two different methodologies for the kidney, liver, parotids, and submandibulars were 5±3, -4±30, 17±18, and 25±17 % respectively. MIM reported higher AD than TORCH in the majority of cases: 89% of organ and 80% lesion cases respectively.
Conclusion
As the field pushes to potentially use dosimetry in clinical routine, differences in reported AD need to be closely evaluated and understood.