• Discussion

    This study has demonstrated that in all untreated NFA and GHA patients tested abnormal TYR-PET uptake is present and PSR can be determined. Our findings therefore suggest, that PET imaging using amino acid as tracer is a feasible technique to document amino acid metabolism in these tumours in vivo. The volume of the TYR-PET uptake decreased parallel with the MRI tumour volume reduction after transsphenoidal surgery. In response to radiation therapy however, the volume of the TYR-PET uptake decreased but the MRI tumour volume did not show any change. These findings support the concept that this technique bears clinical potential to detect changes in biological activity of these tumours.
    Several amino acid tracers, including 11L-C-Methionine have been used so far to detect biological activity of pituitary adenoma. In this study, we used L-[1-11C]-Tyrosine as tracer, because Tyrosine has a higher incorporation into proteins and a lower amount of metabolites than methionine in brain and tumour tissue11, and this tracer is not incorporated to a relevant extent in normal pituitary tissue and in brain tissue, as documented in PET-imaging studies of glioma patients13,14. Furthermore, in animal models a radiation dose-dependent reduction of tracer uptake has been reported, which corre-lated with the changes in tumour volume15. In the present study, its incorporation in NFA and GHA was found to be sufficient for imaging. Thus, all NFA and GHA were well visualized because the high ratio of uptake in the adenoma compared to the surroun-ding structures.
    MRI is considered to be the imaging modality of choice for the diagnosis and follow-up of pituitary disorders, because of its adequate soft tissue contrast4. In our study, there was considerable agreement between TYR-PET and MRI tumour volumes, and tumour volume measured by MRI was not significantly different compared to its TYR-PET volume. It seems that heterogeneity of tracer uptake within adenoma or its remnant was not large enough to result in measurable differences in tumour volume between these imaging techniques. Indeed, in agreement with our hypothesis that
    radiation therapy would affect TYR-PET uptake and/or PSR, we observed that TYR-PET but not MRI tumour volume decreased after radiation therapy, which can be expected to diminish protein metabolism in remaining tumour tissue6. TYR-PET imaging may therefore provide a clinically relevant imaging technique which is complementary to MRI. In particular, the presence of residual pituitary adenoma after surgery has therapeutic consequences1, and our study raises the possibility that abnormal amino acid PET imaging and metabolism in pituitary adenoma after neurosurgical intervention could be helpful to target additional therapy. However, we could not perform fusion of MRI with TYR-PET scanning with the presently used imaging modalities, and it was not possible to document abnormalities in pituitary anatomy precisely with TYR-PET imaging. Finally, our report should be regarded as a proof of concept study. A limitation is its small sample size, which precludes to determine the value of PET in NFA compared to hormone secre-ting tumours, such as GHA, ACTH-producing adenomas and prolactinomas.
    In conclusion, the present preliminary study results suggest that TYR-PET may yield complementary information regarding biological tumour activity in NFA and GHA. The diagnostic value of this tracer technique to predict pituitary adenoma behaviour needs to be validated in a larger long-term study.

Comments are closed.