Validation of simplified dosimetry approaches in ⁸⁹Zr-PET/CT: the use of manual versus semi-automatic delineation methods to estimate organ absorbed doses.
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.