Abstract
Purpose: PET/CT imaging allows for image based estimates of organ and red marrow (RM) residence times. The aim of this study was to derive PET/CT based radiation dosimetry for 89Zr-cetuximab with special emphasis on determining RM absorbed dose. Methods: Seven patients with colorectal cancer received 36.9 ± 0.8 MBq 89Zr-cetuximab within 2 hours after administration of a therapeutical dose of 500 mg•m-2 cetuximab. Whole body PET/CT scans as well as blood samples were obtained 1, 24, 48, 94 and 144 hours post injection. RM activity concentrations were calculated from manual delineation of the lumbar vertebrae and blood samples assuming a fixed red marrow to plasma activity concentration ratio (RMPR) of 0.19 The cumulated activity was calculated as the area under the curve of the organ time-activity data (liver, lungs, kidneys, spleen and red marrow), assuming physical decay after the last scan. The residence time for each organ was derived by dividing the cumulated activity with the total injected activity. The residence time in the remainder of the body was calculated as the maximum possible residence time minus the sum of residence time of source organs, assuming no excretion during the time course of the scans. The (self and total) RM and organ absorbed doses as well as the effective whole body radiation dose were obtained using dose conversion factors from OLINDA/EXM 1.1. Several simplified three time-point dosimetry approaches were also evaluated. Results: Approach a yielded self and total RM doses of 0.17 ± 0.04 and 0.54 ± 0.07 mGy•MBq-1, respectively. Approach b deviated by −21% in self dose and −6% in total dose. RMPR increased over time in 5 out of 7 patients. The highest 89Zr absorbed dose was observed in liver with 2.60 ± 0.78 mGy•MBq-1, followed by kidneys, spleen and lungs, whilst the effective whole body dose was 0.61 ± 0.09 mSv•MBq-1. The simplified three time-point (1, 48 and 144 hr) dosimetry approach deviated by at most 4% in both organ absorbed doses and effective dose. Conclusion: Although total RM dose estimates obtained with the two approaches only differed by at most 6%, image based approach is preferred, as it accounts for non-constant RMPR. The number of successive scans can be reduced to 3 without affecting effective dose estimates.
- Monoclonal Antibodies
- PET/CT
- Radiobiology/Dosimetry
- <sup>89</sup>Zr
- PET
- absorbed dose
- bone marrow
- cetuximab
- Copyright © 2015 by the Society of Nuclear Medicine and Molecular Imaging, Inc.