Page 59 - 89Zr-Immuno-PET:Towards a Clinical Tool to Guide Antibody-based Therapy in Cancer

Immuno-PET with 89Zr-cMab U36 in head and neck cancer
group of patients, a sensitivity of 50% was found for nuclear imaging, the same as
for CT and MRI. Because the number of patients in both studies was small, it is not
justified to compare the performance of 89Zr-immuno-PET and 99mTc-SPECT just
on the basis of sensitivity percentages. Nevertheless, it became clear from the
present study that 89Zr-immuno-PET performs better with respect to tumor delineation. One reason for this is the better spatial resolution of PET. 89Zr- immuno-PET showed detailed delineation of organs like heart as well as of tumors 3 and blood vessels (Figure 1). This had not been the case with 99mTc-SPECT (3).
Another reason for better delineation is the longer half- life of 89Zr than of 99mTc (78.4 versus 6.0 hours), which allowed imaging at later time points, when tumor- to-nontumor radioactivity uptake ratios are higher (Figure 1). Indeed, in the present study delineation of primary tumors and lymph node metastases was better at later time points (72 and 144 hours) than at earlier time points (data not shown). Image quality might be further improved by elongation of the scanning time for the two bed positions covering the head and neck region.
Although good resolution is an advantage, it is fair to say that immuno-PET is hampered by the lack of anatomic structures. For example, because of the slow clearance of the conjugate from the blood, it might be difficult to distinguish a targeted lymph node metastasis from a cross-section through a blood vessel. In the present study, no explanation was found for the two false-positive observations with 89Zr-immuno-PET. An improvement in image interpretation might be obtained by fusion of immuno-PET images with CT or MRI images. This approach enables the combination of (tumor) biological and anatomic information. We explored this approach for a few patients, including patient 16 with extensive bilateral lymph node involvement (Figure 2). Figure 4 shows the fused PET-CT image of the same patient 16, and the fusion makes clear that increased uptake of 89Zr-cMAb U36 indeed is confined to the enlarged lymph nodes in the neck. To get a proper match between the immuno-PET and CT images, the head of this patient had to be fixed in the same way during both imaging procedures by using a thermoplastic radiotherapy mask. Problems related to matching will be solved when PET imaging and anatomical CT or MRI imaging are performed simultaneously by use of hybrid scanners. The use of combined PET/CT scanners is rapidly expanding and first efforts in developing MRI-compatible PET scanners have been reported (16). The availability of anatomic data will not only provide landmarks for PET image interpretation, but can also be used for more accurate quantification of MAb distribution (e.g., partial volume correction).
57