A within patient comparison of 11C-methionine (Met) and 18F-fluoroethyltyrosine (FET) PET for the imaging of pituitary tumours - a pilot study

James MacFarlane; Daniel Gillett; August Palma; Graham Smith; Victoria Warnes; Franklin Aigbirhio; Jonathan Jones; Heok Cheow; Waiel Bashari; Mark Gurnell

doi:10.1530/endoabs.109.OC7.5

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Endocrine Abstracts (2025) 109 OC7.5 | DOI: 10.1530/endoabs.109.OC7.5

SFEBES2025 Oral Communications Neuroendocrinology and Pituitary (6 abstracts)

A within patient comparison of 11C-methionine (Met) and 18F-fluoroethyltyrosine (FET) PET for the imaging of pituitary tumours – a pilot study

James MacFarlane ¹ , Daniel Gillett ¹ , August Palma ¹ , Graham Smith ² , Victoria Warnes ³ , Franklin Aigbirhio ² , Jonathan Jones ¹ , Heok Cheow ¹ , Waiel Bashari ¹ & Mark Gurnell ¹

Author affiliations

¹Cambridge Endocrine Molecular Imaging Group, University of Cambridge and Cambridge University Hospitals NHS Foundation Trust, Cambridge, United Kingdom; ²Wolfson Brain Imaging Centre, Cambridge, United Kingdom; ³PET-CT unit, Cambridge University Hospitals NHS Foundation Trust, Cambridge, United Kingdom

Background: Pituitary MRI does not always reliably identify sites of de novo, residual or recurrent pituitary adenoma (PA). Some patients are therefore denied potentially curative intervention (e.g. transsphenoidal surgery or stereotactic radiosurgery) or undergo suboptimal treatment because of the absence of a clear target. Molecular imaging can complement anatomical imaging to aid tumour localisation. To date, [11C]methionine PET (Met-PET), is the preferred radiotracer for pituitary imaging. However, with a short half-life (20 min) access is limited to centres with an on-site cyclotron. Recently, [18F]fluoroethyltyrosine (FET-PET) (half-life 110 min) has been proposed as an alternative amino acid radionuclide, which could increase access to functional pituitary imaging.

Methods: We have conducted the first within patient comparison of [11C]Met-PET and [18F]FET-PET across a spectrum of pituitary tumour subtypes. In the initial pilot phase, 4 corticotropinomas, 3 thyrotropinomas, 2 somatotropinomas and 1 prolactinoma (with either de novo or residual disease) were assessed. Five patients had suspected cavernous sinus extension (CSE). Both qualitative and quantitative [standardised uptake values (SUV) normalised to cerebellum] tumoral uptake were assessed.

Results: Although all pituitary tumour subtypes showed focal [18F]FET uptake, in all cases PA were more readily localised [both qualitatively (visual inspection) and quantitatively (SUVmax)] using [11C]Met-PET. Importantly, unlike [11C]Met, significant physiological uptake of [18F]FET was observed in the normal CS in all patients, which resulted in tumoral CSE being less readily appreciated when compared with [11C]Met-PET.

Conclusions: [18F]FET-PET can aid localisation of all functioning PA subtypes. However, our findings suggest that tumour-to-background uptake ratios are superior for [11C]Met-PET, especially when there is CSE. Therefore, whilst [18F]FET-PET may facilitate increased access to molecular pituitary imaging, [11C]Met-PET is likely to remain the radiotracer of choice for the most challenging cases with low volume disease and/or CSE.