RESUMO
BACKGROUND: The study aims to develop and validate an automatic delineation method for estimating red bone marrow (RM) activity concentration and absorbed dose in (89)Zr positron emission tomography/computed tomography (PET/CT) studies. Five patients with advanced colorectal cancer received 37.1 ± 0.9 MBq [(89)Zr] cetuximab within 2 h after administration of a therapeutic dose of 500 mg m(-2) unlabelled cetuximab. Per patient, five PET/CT scans were acquired on a Gemini TF-64 PET/CT scanner at 1, 24, 48, 96 and 144 h post injection. Low dose CT data were used to manually generate volumes of interest (VOI) in the lumbar vertebrae (LV). In addition, LV VOI were generated automatically using an active contour method in a low dose CT. RM activity was then determined by mapping the low dose CT-derived RM VOI onto the corresponding PET scans. Finally, these activities were used to derive residence times and, subsequently, the self and total RM absorbed doses using OLINDA/EXM 1.1. RESULTS: High correlations (r (2) > 0.85) between manual and automated VOI methods were obtained for both RM activity concentrations and total absorbed doses. On average, the automatic method provided values that were lower than 5 % compared to the manual method. CONCLUSIONS: An automated and efficient VOI method, based on an active contour approach, was developed, enabling accurate estimates of RM activity concentrations and total absorbed doses.
RESUMO
PURPOSE: Increasing interest in immuno-positron emission tomography (PET) studies requires development of dosimetry methods which will provide accurate estimations of organ absorbed doses. The purpose of this study is to develop and validate simplified dosimetry approaches for (89)Zirconium-PET (Zr-PET)/computed tomography (CT) studies. METHODS: Five patients with advanced colorectal cancer received 37.1 ± 0.9 MBq (89)Zr-cetuximab within 2 h after administration of a therapeutic dose of 500 mg m(-2) cetuximab. PET/CT scans were obtained 1, 24, 48, 94, and 144 h post injection. Volumes of interest (VOIs) were manually delineated in lungs, liver, spleen, and kidneys for all scans, providing a reference VOI set. Simplified manual VOIs were drawn independently on CT scans using larger voxel sizes. The transformation of VOIs based on rigid and/or nonrigid registrations of the first CT scan (CT1) onto all successive CT scans was also investigated. The transformation matrix obtained from each registration was applied to the manual VOIs of CT1 to obtain VOIs for the successive scans. Dice similarity coefficient (DSC) and Hausdorff distance were used to assess the performance of the registrations. Organ total activity, organ absorbed dose, and effective dose were calculated for all methods. RESULTS: Semi-automatic delineation based on nonrigid registration showed excellent agreement for lungs and liver (DSC: 0.90 ± 0.04; 0.81 ± 0.06) and good agreement for spleen and kidneys (DSC: 0.71 ± 0.07; 0.66 ± 0.08). Hausdorff distance ranged from 13 to 16 mm depending on the organ. Simplified manual delineation methods, in liver and lungs, performed similarly to semi-automatic delineation methods. For kidneys and spleen, however, poorer accuracy in total activity and absorbed dose was observed, as the voxel size increased. Organ absorbed dose and total activity based on nonrigid registration were within 10%. The effective dose was within ±3% for all VOI delineation methods. CONCLUSIONS: A fast, semi-automatic, and accurate delineation method based on nonrigid registration was developed for determination of organ absorbed and effective dose in (89)Zr-PET/CT studies which may also be applied to other long-lived radionuclide PET/CT studies.
