Toward the standardization of radiopharmaceutical therapies: a technical note evaluating a clinical dosimetry workflow for single-time-point 177Lu SPECT/CT-based therapies.

Abstract

Purpose: The lack of standardized dosimetry workflows in lutetium-177 ( $_{}^{177}$ Lu) radiopharmaceutical therapies results in inconsistent absorbed doses and limits treatment planning. This study aims to evaluate the accuracy and variability of a single-time-point ¹⁷⁷Lu SPECT/CT commercial workflow to help harmonize its protocol.

Methods: The dosimetry workflow evaluated in this study predominatly followed that of MIM SurePlan^TM MRT. ¹⁷⁷Lu SPECT/CT images of a Jaszczak and a NEMA phantoms were acquired in GE 670 DR scanner. Absorbed dose (Gy/MBq/s) was calculated in the background and sphere inserts with varied reconstruction iterations, calibrations, voxel-based dosimetry methods, and target volume segmentations. Ground truth absorbed doses were created using CT images and voxel S-value (VSV) water kernels. The validity of the density-corrected (DC) kernel for use in ground-truth dosimetry evaluations was further investigated. The accuracy and variability of the dosimetry workflow were evaluated using percent error and the coefficient of variation (CV) of mean absorbed doses.

Results: Mean absorbed dose accuracy improved for both the voxel-based VSV and local deposition (LD) methods until 480 equivalent iterations for all target volumes. DC kernel was found viable for creating reference absorbed doses. The calibration CV was 5.18% when phantom and calibration regions were varied. The VSV method demonstrated absorbed doses that were 10 to 150% higher than those calculated with the LD method. The overall variability in absorbed dose reached up to 84% when reconstruction, calibration, dosimetry, and segmentation methods were varied.

Conclusions: A single dosimetry workflow has demonstrated markedly large variability in absorbed dose accuracy. By evaluating the accuracy of absorbed dose, our study helped to propose a harmonized MIM SurePlan^TM MRT workflow for single-time-point $_{}^{177}$ Lu SPECT/CT-based therapies.