Author: Alejandro Bertolet, Mislav Bobić, Carlos Huesa-Berral, Aileen O'Shea, Ralph Weissleder 👨🔬
Affiliation: Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Massachusetts General Hospital and Harvard Medical School 🌍
Purpose: To develop a pipeline for estimating the absorbed dose to various organs from positron emission tomography (PET) combined with magnetic resonance imaging (MRI). We use this pipeline to evaluate the radiological safety of new PET tracers such as 64Cu-Macrin for detecting macrophage accumulation in cancer and other inflammatory conditions.
Methods: Absorbed doses are retrospectively calculated for five healthy volunteers enrolled in a clinical trial investigating the clinical safety of 64Cu-Macrin. Each patient was administered 64Cu-Macrin at a mass dose of <100 μg by intravenous injection. Two whole-body PET/MRI scans were acquired for each patient: the initial scan at 0 hours (within 10 minutes post-injection) and a second scan at 24 hours post-injection. Voxel S-values were simulated for the 64Cu PET tracer in soft tissue using the OpenTOPAS Monte Carlo tool, from which dose rate distributions were calculated for each PET scan according to the Medical Internal Radiation Dosimetry (MIRD) formalism. Contours of ~50 major anatomical structures were acquired on the MRI images using the TotalSegmentator tool. For each structure, mean dose rates were calculated at two time points corresponding to the timing of the PET scans. Time-dose-rate curves were derived, allowing integration over time to calculate the total absorbed dose from the PET tracer in each organ.
Results: Organs receiving the highest absorbed doses, in descending order, were the liver, urinary bladder, gallbladder, kidneys, and spleen. The mean absorbed dose to these organs ranged from 10.3 mGy to 4.0 mGy, with a maximum of 26.4 mGy to the liver of one patient, which is within the range of well-tolerable doses and indicates a safe profile of the investigated drug.
Conclusion: We developed a dosimetry tool for patients undergoing PET/MRI to assess the radiological safety of 64Cu-Macrin PET. This tool allows automated yet sophisticated dosimetric estimations for diagnostic PET tracers.