• Introduction

    In view of the high recurrence rate in case of residual pituitary adenoma after surgery alone the accurate evaluation of post-operative residual pituitary adenoma by ima-ging techniques is essential1,2. The detection of residual pituitary adenoma after surgery by magnetic resonance imaging (MRI) and/or computed tomography (CT) is frequently hampered by postoperative tissue remodelling3,4. It is particularly difficult to distinguish residual vital adenoma tissue from operative changes in sellar structures, and to discriminate between vital and non vital tissue after radiation therapy. Moreover, it is obvious that the diagnostic yield of hormonal assessment is mostly restricted to those tumours which abnormally secrete anterior pituitary hormones, like growth hormone, adrenocorticotropic hormone and prolactin. Therefore an additional imaging technique is necessary which enables to document the biological behaviour of the tumour.
    Pituitary adenomas are characterized by a high amino acid metabolism5-8. Consequently, positron emission tomography (PET) with radio labeled amino acids may be a suitable method for accurate detection of the activity of these tumours in response to medical and surgical treatment and radiation therapy. Accordingly, it has been shown in prolactinoma and growth hormone producing pituitary adenoma (GHA)9, that amino acid metabolism changes in response to medical treatment5, but treatment responses after surgery and radiation therapy in NFA and GHA have not been reported so far.
    L-[1-11C]-tyrosine is suitable for calculation of protein synthesis rates, since this tracer has a small pool of free tyrosine in plasma and tissue and a rapid high incorporation into protein, which is not hampered by the blood brain barrier. This allows not only visualization but also quantification of the protein synthesis rate (PSR), which is higher in metabolically active and proliferating pituitary adenoma than in normal brain tissue5,10,11.
    We hypothesized that after surgical tumour reduction TYR-PET tumour volume would decrease parallel with MRI tumour volume changes, without affecting PSR. In view to the efficacy of radiation therapy to prevent re-growth of residual pituitary ade-noma1, we expected that this treatment would, in contrast, affect biological tumour acti-vity, as determined by TYR-PET tumour volume and/or PSR. The present study was therefore initiated to demonstrate the feasibility of pituitary TYR-PET imaging and measure-ment of protein synthesis rate after neurosurgical intervention and in response to radiation therapy.

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