In vivo quantification of tumoral radioactivity by microPET: Direct comparison with ex vivo gamma counting

Abstract

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Objectives: Dedicated small animal PET (microPET) allows image-based quantification of the tumoral radioactivity in vivo. We compared the measurements of the tumoral radioactivity obtained by the microPET and ex vivo gamma counting. Methods: SCID mice with subcutaneous M-14/High Molecular Weight-Melanoma Associated Antigen (HMW-MAA)+ve melanoma xenograft (n=12) and C57BL/6 mice with intracerebral GL261 glioma graft (n=14) were injected with 100 µCi of HMW-MAA specific I-124 labeled monoclonal antibodies, 763.74 and VT68.2, respectively. Mice were imaged with a MicroPET Focus 120® scanner 4 to 6 days after the injection. Within 1 hour of the microPET scan, mice were sacrificed and tumors were dissected out. Reconstructed images were quantified using in-built ASIPRO ® software executed on an IDL Virtual Machine 6.0 platform. Region of interest (ROI) was drawn around the tumor on decay corrected whole-body coronal images. A calibration factor, derived from the scanning of a cylindrical phantom of known volume and radoactivity, was applied to convert ROI counts to the percentage of the injected dose per gram (%ID/g) of tumor tissue. For ex vivo gamma counting, tumor specimens were weighed and counted for 1 minute using an automated gamma counter in reference to the counts of standard samples prepared from aliquots of the injected doses. Results: Tumoral radioactivity (%ID/g) obtained by the microPET correlated well with that obtained by the gamma counting (r=0.83, P<0.0001); however, microPET measurements were significantly higher (P=0.003). Bland-Altman plot revealed a trend of systematic overestimation of gamma counting measurements by the microPET (r=0.65, P=0.0002). When measurements were divided into a high count group and a low count group, the agreement between 2 modalities was better in the former group. Conclusions: MicroPET measurements of tumoral radioactivity correlated well with that obtained by ex vivo gamma counting. However, MicroPET tended to overestimate the radioactivity measurements that were obtained by direct gamma counting of the tumor specimens.

Research Support (if any): None