A robotic system for 18F-FMISO PET-guided intratumoral pO2 measurements.

An image-guidedrobotic system was used to measure the oxygen tension ( p O 2 ) in rodent tumor xenografts using interstitial probes guided by tumor hypoxia PETimages. Rats with ∼ 1 cm diameter tumors were anesthetized and immobilized in a custom-fabricated whole-body mold. Imaging was performed using a dedicated small-animal PET scanner (R4 or Focus 120 microPET™) ∼ 2 h after the injection of the hypoxia tracer F 18 -fluoromisonidazole ( F 18 -FMISO). The coordinate systems of the robot and PET were registered based on fiducial markers in the rodent bed visible on the PETimages. Guided by the 3D microPET image set, measurements were performed at various locations in the tumor and compared to the corresponding F 18 -FMISO image intensity at the respective measurement points. Experiments were performed on four tumor-bearing rats with 4 (86), 3 (80), 7 (162), and 8 (235) measurement tracks (points) for each experiment. The F 18 -FMISO image intensities were inversely correlated with the measured p O 2 , with a Pearson coefficient ranging from − 0.14 to − 0.97 for the 22 measurement tracks. The cumulative scatterplots of p O 2 versus image intensity yielded a hyperbolic relationship, with correlation coefficients of 0.52, 0.48, 0.64, and 0.73, respectively, for the four tumors. In conclusion, PETimage-guided p O 2 measurement is feasible with this robot system and, more generally, this system will permit point-by-point comparison of physiological probe measurements and image voxel values as a means of validating molecularly targeted radiotracers. Although the overall data fitting suggested that F 18 -FMISO may be an effective hypoxia marker, the use of static F 18 -FMISO PET postinjection scans to guide radiotherapy might be problematic due to the observed high variation in some individual data pairs from the fitted curve, indicating potential temporal fluctuation of oxygen tension in individual voxels or possible suboptimal imaging time postadministration of hypoxia-related trapping of F 18 -FMISO.