Image-Based Red Marrow Dosimetry in Patients Undergoing [177Lu]Lu-PSMA Radiopharmaceutical Therapy

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Ziel/Aim: Red marrow irradiation is a major concern for patients with mCRPC undergoing [ 177 Lu]Lu-PSMA therapy and monitoring the red marrow absorbed dose is recommended to minimise side effects. The aim of this study was to develop an automated workflow for full image-based red marrow dosimetry. Methodik/Methods: In the first part of the study, 175 virtual patient phantoms with realistic activity distributions were generated based on 639 pre-therapeutic [ 18 F]F-PSMA-1007 PET/CT scans. The activity concentrations used were extracted from real SPECT scans acquired at 24 h p.i after the first cycle with 7.4 GBq [ 177 Lu]Lu-PSMA-I&T (bone lesion:kidney:background ratio of 40:20:1). SIMIND Monte Carlo tool was used to simulate the SPECT acquisition using the virtual phantoms. A recovery coefficient (RC) analysis was conducted for skeletal VOIs with and without bone lesions. In the second part of the study, red marrow self-absorbed doses were estimated from real patient data. First cycle treatments of 11 patients ([ 177 Lu]Lu-PSMA-I&T) were analysed. CT segmentation was performed using Totalsegmentator [ 1 ], followed by lesion segmentation using the Otsu thresholding method. Activity estimation was performed in all the bones available in Totalsegmentator (61 skeletal VOIs) without segmented lesions. The red marrow mass was estimated from the volume fraction of red marrow based on the modified ICRP 89 male phantom [ 2 ] and CT-based density. Ergebnisse/Results: The simulation study shows a severe overestimation of activity (median RC: 337%, min: 44%, max: 1788%) in skeletal regions with lesions, while the median RC in skeletal VOIs without lesions was 99% (min: 26%, max: 203%). The median red marrow self-absorbed dose was 3 mGy/GBq (min/max: 2/65 mGy/GBq). Schlussfolgerungen/Conclusions: This study presents an automated workflow to estimate red marrow self-absorbed doses. Without compensation for limited spatial resolution and depending on the clinical patient case, red marrow self-absorbed doses should be extracted from skeletal sites without lesions. Future applications of this workflow will include a larger patient cohort and correlation with clinical outcome. Publication History Article published online: 12 March 2025 © 2025. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany

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