Purpose: to assess the utility of (18F)fluoroethyl-l-tyrosine positron emission tomography co-registered with magnetic resonance imaging (FET-PET/MRICR) in patients with difficult to localize prolactinoma to inform clinical decision making and treatment planning.

Methods: retrospective cohort study of 17 consecutive patients with complex prolactinoma undergoing FET-PET/MRICR between October 2020 and August 2022 for (1) additional information in case of difficult remnants after transsphenoidal surgery (TSS) or pharmacological treatment or (2) diagnosis in absence of a (clear) adenoma on conventional MRI at diagnosis or after treatment.

Results: FET-PET/MRICR identified a lesion in 14/17 patients, yet failed to identify active lesions in 2 patients with negative conventional MRI but prolactin >7.5 times upper limit of normal, and FET-PET/MRICR results were inconclusive in 1 patient due to diffuse tracer uptake 10 weeks postoperative. Foci on FET-PET/MRICR corresponded completely with the lesion on conventional MRI in 10 patients, partially in 3 patients and new foci were identified in 4 patients. Functional imaging  influenced clinical decision making in 12/17 patients: 7 patients underwent TSS and 5 patients did not. One patient underwent surgery despite negative FET-PET/MRICR due to a high need for alternative treatment, 3 did not undergo TSS despite respectable FET-PET/MRICR foci and 1 patient underwent additional diagnostics due to inconclusive FET-PET/MRICR results. Perioperative findings corresponded with FET-PET/MRICR in 5/8 and histopathology was positive in 6/8, achieving the treatment goal in 7/8 patients and remission in 5/7 patients in whom total resection was intended.

Conclusion: FET-PET/MRICR can be of added value  in the preoperative decision-making process  for selected patients with difficult to localize prolactinoma (remnants), or patients lacking a substrate on conventional MRI.