Cushing's disease results from adrenocorticotropic hormone (ACTH) secretion by corticotroph pituitary neuroendocrine tumors (PitNETs). Magnetic resonance imaging (MRI) is the reference modality for etiological diagnosis but may to visualize small corticotroph microadenomas in up to 30% of the cases, and false positives may occur.

The study hypothesis is that positron emission tomography (PET) using [18F]fluoroethyl-L-tyrosine ([18F]FET) improves localization of ACTH-secreting corticotroph microadenomas compared with MRI and could inform surgical planning and reduce reliance on invasive inferior petrosal sinus sampling.

This observational cohort (retrospective and prospective data) will assess the diagnostic performance of [18F]FET PET for tumor localization using postoperative histopathology as the gold standard.

Secondary aims include:

1. assessing cases in which PET modifies localization relative to MRI and is correct by gold standard;
2. inter-reader agreement between two nuclear medicine physicians;
3. correlations between PET signal and biochemical markers of hypercortisolism
4. uni- and multivariable analyses of clinical and imaging parameters influencing PET results;
5. association between PET findings and subsequent biological remission.

Inclusion Criteria:

• Adults (≥18 years).
• Biochemical diagnosis of Cushing's disease as part of initial management.
• Pituitary MRI performed; if visible, microadenoma <10 mm in diameter.
• Indication for discussion in multidisciplinary tumor board for surgical management.

Exclusion Criteria:

• Minor (age <18 years).
• Macroadenoma of the pituitary.
• ACTH-dependent hypercortisolism secondary to ectopic secretion.

Adults treated at a tertiary academic hospital with biochemically confirmed Cushing's disease, evaluated by pituitary MRI and [18F]FET PET as part of diagnostic work-up, and discussed for surgery.