Inter-observer agreement for tumor uptake quantification of 89Zr-labeled anti-CD20 antibodies with PET
The Journal of Nuclear Medicine(2016)
Abstract
1412 Objectives Quantification of tumor uptake of 89Zirconium(89Zr)-labeled monoclonal antibodies on immuno-positron emission tomography (immuno-PET) is usually based on max or peak activity concentrations (AC) in areas of focally enhanced uptake. These measures reflect the highest uptake only. In contrast, manual delineation allows for characterization of uptake in the whole tumor area in terms of mean AC, standard deviation and volume. However, manual delineation is more observer dependent.
The aim of this study was to assess inter-observer agreement for quantification of tumor uptake of 89Zr-labeled anti-CD20 antibodies for max, peak and mean AC.
Methods Six patients with non-Hodgkin lymphoma received an imaging dose of 74 MBq 89Zr-anti-CD20 antibodies. Whole body PET scans were obtained, 3 and 6 days post injection (D3 and D6). Tumor lesions, defined as focally enhanced, non-physiological uptake, were identified by an experienced nuclear medicine physician. 3 observers independently quantified tumor uptake for all identified tumor lesions. 2 observers had previous experience with manual delineation of tumors on immuno-PET. Tumors were delineated on immuno-PET using a standard operating procedure, while observers had access to baseline FDG-PET and low dose CT (LDCT) for confirmation of delineation.
Max, peak and mean AC per VOI were derived. Coefficients of variation (COV; defined as standard deviation over the observers divided by the average over the observers) were calculated for D3 and D6.
Results Visual assessment identified a total of 27 tumor lesions in five patients. In one patient no tumor uptake was visible. Inter-observer agreement for ACmax and ACpeak was high, resulting in a low COV (COV for ACmax and ACpeak within 1% ± 3%). ACmean derived from manual delineation was more observer-dependent with a COV of 4 ± 3% on D3 and 8 ± 5 % on D6.
Conclusions In order to quantify uptake in the whole tumor area, manual delineation is needed. This method shows an inter-observer agreement within 8% for tumor uptake of 89Zr-anti-CD20 antibodies. Quantification of only the highest tumor uptake, using ACmax or peak, results in even better inter-observer agreement (COV within 1%).
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