Methods: Biopsies were collected from individual tumors between approximately 3 and 25 hours after the intravenous injection of 64Cu-ATSM and pimonidazole. 64Cu-ATSM autoradiography and pimonidazole immunostaining was performed on sectioned biopsies. Acquired 64Cu-ATSM autoradiography and pimonidazole images were rescaled, aligned and their distribution patterns compared. 64Cu-ATSM and 18FDG PET/CT scans were performed in a concurrent study and uptake characteristics were obtained for tumors where available.
Results: Maximum pimonidazole pixel value and mean pimonidazole labeled fraction was found to be strongly correlated to 18FDG PET uptake levels, whereas more varying results were obtained for the comparison to 64Cu-ATSM. In the case of the latter, uptake at scans performed 3 h post injection (pi) generally showed strong positive correlated to pimonidazole uptake.Comparison of distribution patterns of pimonidazole immunohistochemistry and 64Cu-ATSM autoradiography yielded varying results. Significant positive correlations were mainly found in sections displaying a heterogeneous distribution of tracers.Conclusions:Tumors with high levels of pimonidazole staining generally displayed high uptake of 18FDG and 64Cu-ATSM (3 h pi.). Similar regional distribution of 64Cu-ATSM and pimonidazole was observed in most heterogeneous tumor regions. However, tumor and hypoxia level dependent differences may exist with regard to the hypoxia specificity of 64Cu-ATSM in canine tumors.