The results of a study on the role of Tyrosine-PET (TYR-PET) for pituitary adenoma imaging after surgery and radiation therapy were reported in Chapter 7. TYR-PET is able to visualize protein synthesis in tissues. TYR-PET imaging was compared with magnetic resonance imaging (MRI), which is currently the standard imaging modality in pituitary adenomas. In 9 NFA-patients and 3 GHA-patients TYR-PET and MRI images were compared pre- and postoperatively. The results showed that tumour volume reduction of the pituitary adenoma after surgery was similar when determined by both imaging modalities. In six of these patients (4 NFA- and 2 GHA –patients) TYR-PET was used to document residual activity three years after completion of radiation therapy. In these patients the MRI-volumes remained unchanged in contrast to TYR-PET imaging, that revealed a median reduction of 50%. Likewise, there was also a trend towards reduced protein synthesis in the residual pituitary adenoma 3 years after completion of radiation therapy. These preliminary results support the concept that it is possible with TYR-PET imaging to follow biological tumour activity of pituitary adenoma, and suggest that this imaging technique may yield additional information compared to MRI.