11C-methionine PET aids localization of microprolactinomas in patients with intolerance or resistance to dopamine agonist therapy

The Use of [11C]C-Methionine in Diagnostics of Endocrine Disorders with Focus on Pituitary and Parathyroid Glands

The rapid development of nuclear medicine offers vast opportunities for diagnosing neoplasms, particularly in endocrinology. The use of the [11C]C-methionine radiotracer is currently limited due to its physical properties and the complex production process. However, studies conducted so far have demonstrated its utility in PET imaging, helping to detect lesions that often remain elusive with other modalities. This systematic review focuses on [11C]C-methionine in diagnosing hyperparathyroidism and pituitary tumors, highlighting its high effectiveness, which can be crucial in diagnosis. Despite some disadvantages, it should be considered when available, especially when other modalities or radiotracers fail.

Clinical use of [18F]fluoro-ethyl-L-tyrosine PET co-registered with MRI for localizing prolactinoma remnants

PURPOSE

To assess the utility of [18F]fluoroethyl-L-tyrosine PET co-registered with magnetic resonance imaging ([18F]FET-PET/MRICR) in patients with difficult-to-localize prolactinoma to inform clinical decision-making and (surgical) treatment planning.

METHODS

Retrospective cohort study of 17 consecutive patients with prolactinoma undergoing [18F]FET-PET/MRICR between October 2020 and September 2022 for either (1) additional information in case of difficult-to-visualize remnants after prior transsphenoidal surgery (TSS), or pharmacological treatment, or (2) radiological diagnosis in absence of a (clear) adenoma on diagnostic/post-treatment conventional MRI.

RESULTS

[18F]FET-PET/MRICR identified a lesion in 14/17 patients, yet failed to identify active lesions in 2 patients with negative conventional MRI despite prolactin > 7.5 times upper limit of normal. [18F]FET-PET/MRICR results were inconclusive in 1 patient due to diffuse tracer uptake 10 weeks post-surgery. [18F]FET-PET/MRICR was completely concordant with a suspected lesion on conventional MRI in 10/17 patients, and partially concordant in 3/17 patients. New foci were identified in 4/17 patients. The [18F]FET-PET/MRICR conclusions influenced clinical shared decision-making in 15/17 patients, of whom 7 patients underwent TSS and 8 refrained from TSS. One patient underwent TSS despite negative [18F]FET-PET/MRICR, and one patient underwent additional imaging. Intraoperative findings corresponded with [18F]FET-PET/MRICR in 5/8 patients, and immunohistochemistry was positive in 5/8 patients. The treatment goal was achieved in 7/8 patients, and remission was achieved in 5/7 patients in whom total resection was considered feasible.

CONCLUSION

[18F]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.

18F-fluoro-ethyl-tyrosine PET co-registered with MRI in patients with persisting acromegaly

OBJECTIVE

To report our experience with 18F-fluoro-ethyl-tyrosine (FET) positron emission tomography-computed tomography (PET-CT) co-registered with magnetic resonance imaging (MRI) (FET-PET/MRICR) in the care trajectory for persistent acromegaly.

DESIGN

Prospective case series.

PATIENTS

Ten patients with insufficiently controlled acromegaly referred to our team to evaluate surgical options.

MEASUREMENTS

FET-PET/MRICR was used to support decision-making if MRI alone and multidisciplinary team evaluation did not provide sufficient clarity to proceed to surgery.

RESULTS

FET-PET/MRICR showed suspicious (para)sellar tracer uptake in all patients. In five patients FET-PET/MRICR was fully concordant with conventional MRI, and in one patient partially concordant. FET-PET/MRICR identified suggestive new foci in four other patients. Surgical re-exploration was performed in nine patients (aimed at total resection (6), debulking (2), diagnosis (1)), and one patient underwent radiation therapy. In 7 of 9 (78%) operated patients FET-PET/MRICR findings were confirmed intraoperatively, and in six (67%) also histologically. IGF-1 decreased significantly in eight patients (89%). All patients showed clinical improvement. Complete biochemical remission was achieved in three patients (50% of procedures in which total resection was anticipated feasible). Biochemistry improved in five and was unchanged in one patient. No permanent complications occurred. At six months, optimal outcome (preoperative intended goal achieved without permanent complications) was achieved in six (67%) patients and an intermediate outcome (goal not achieved, but no complications) in the other three patients.

CONCLUSIONS

In patients with persisting acromegaly without a clear surgical target on MRI, FET-PET/MRICR is a new tracer to provide additional information to aid decision-making by the multidisciplinary pituitary team.

How often should we perform magnetic resonance imaging (MRI) for the follow-up of pituitary adenoma?

Magnetic resonance imaging (MRI) is the examination of choice for diagnosing and monitoring pituitary adenoma (also known as pituitary neuroendocrine tumor or PitNET), whether treated or not. However, repeating the examination too often (and sometimes unnecessarily) is costly, and worrying data on tissue accumulation (brain, bone, etc.) of gadolinium atoms dissociated from their carrier molecule (chelator) have led European authorities to ban contrast agents based on linear chelators of gadolinium, which are particularly susceptible to rapid dissociation, in favor of chemically more stable macrocyclic chelators. It is therefore important to determine the optimal frequency for pituitary MRI monitoring in order to safely assess the natural history or therapeutic response of pituitary adenomas. The aim of this article is to summarize the most recent data on optimal follow-up intervals depending on the type, size and location of the pituitary tumor and the clinical situation in general, in order to generate monitoring algorithms to guide clinicians.

Real-world experience with 11C-methionine positron emission tomography in the management of acromegaly

BACKGROUND

L-[methyl-11C]-methionine-positron emission tomography (Met-PET) is a potentially important imaging adjunct in the diagnostic workup of pituitary adenomas, including somatotroph tumors. Met-PET can identify residual or occult disease and make definitive therapies accessible to a subgroup of patients who would otherwise require lifelong medical therapy. However, existing data on its use are still limited to small case series. Here, we report the largest single-center experience (n = 61) in acromegaly.

METHODS

A total of 189 cases of acromegaly were referred to our national Met-PET service in the last 12 years. For this analysis, we have reviewed outcomes in those 61 patients managed exclusively by our multidisciplinary team (single center, single surgeon). Referral indications were as follows: indeterminate magnetic resonance imaging (MRI; n = 38, 62.3%), occult residual (n = 14, 23.0%), (radio-)surgical planning (n = 6, 9.8%), and occult de novo tumor (n = 3, 4.9%).

RESULTS

A total of 33/61 patients (54.1%) underwent PET-guided surgery. Twenty-four of 33 patients (72.7%) achieved complete biochemical remission following (re-)surgery. Insulin-like growth factor 1 levels were reduced to <2 × upper limit of normal (ULN) in 6 of the remaining 9 cases, 3 of whom achieved levels of <1.1 × ULN compared with mean preoperative levels of 2.4 × ULN (SD 0.8) for n = 9. Only 3 patients developed single new hormonal deficits (gonadotropic/thyrotropic insufficiency). There were no neurovascular complications after surgery.

CONCLUSION

In patients with persistent/recurrent acromegaly or occult tumors, Met-PET can facilitate further targeted intervention (surgery/radiosurgery). This led to complete remission in most cases (24/33) or significant improvement with comparatively low risk of complications. L-[methyl-11C]-methionine-positron emission tomography should therefore be considered in all patients who are potential candidates for further surgical intervention but present no clear target on MRI.

Diagnosis and management of prolactin-secreting pituitary adenomas: a Pituitary Society international Consensus Statement

This Consensus Statement from an international, multidisciplinary workshop sponsored by the Pituitary Society offers evidence-based graded consensus recommendations and key summary points for clinical practice on the diagnosis and management of prolactinomas. Epidemiology and pathogenesis, clinical presentation of disordered pituitary hormone secretion, assessment of hyperprolactinaemia and biochemical evaluation, optimal use of imaging strategies and disease-related complications are addressed. In-depth discussions present the latest evidence on treatment of prolactinoma, including efficacy, adverse effects and options for withdrawal of dopamine agonist therapy, as well as indications for surgery, preoperative medical therapy and radiation therapy. Management of prolactinoma in special situations is discussed, including cystic lesions, mixed growth hormone-secreting and prolactin-secreting adenomas and giant and aggressive prolactinomas. Furthermore, considerations for pregnancy and fertility are outlined, as well as management of prolactinomas in children and adolescents, patients with an underlying psychiatric disorder, postmenopausal women, transgender individuals and patients with chronic kidney disease. The workshop concluded that, although treatment resistance is rare, there is a need for additional therapeutic options to address clinical challenges in treating these patients and a need to facilitate international registries to enable risk stratification and optimization of therapeutic strategies.

Molecular Imaging of Pituitary Tumors

Tumors of the pituitary gland, although mostly benign adenomas, are a cause of significant morbidity and even excess mortality due to local compressive effects (eg visual loss, hypopituitarism) and unregulated hormone secretion (eg acromegaly or Cushing Disease). Surgery, radiotherapy, and medical management (sometimes in combination) may be needed to mitigate the effects of tumor expansion and endocrine dysfunction. Magnetic resonance imaging (MRI) plays a central role in treatment planning for most patients. However, it does not always reliably identify the site(s) of primary or recurrent disease, especially where post-treatment remodeling results in indeterminate anatomical appearances. In these contexts, molecular imaging is a potential game-changer, allowing precise localization of sites of active disease and enabling safe and effective targeted intervention when patients would otherwise be consigned to expensive life-long medication. For pituitary and parasellar imaging, PET is the preferred modality due to its superior spatial resolution and sensitivity compared with SPECT, and an array of PET radioligands have been studied in different pituitary adenoma (PA) subtypes. While 18F-fluorodeoxyglucose (18F-FDG) is widely available, significant heterogeneity in tumoral uptake has limited its use. Instead, ligands targeting specific molecular pathways relevant to PA biology (eg somatostatin or dopamine receptor expression, amino acid uptake) are increasingly preferred and are beginning to find application in routine clinical practice. In addition, novel approaches to distinguish adenomatous tissue from normal gland (eg through comparison of images obtained with different radiotracers) and increase confidence that a suspected abnormal focus is indeed pathological (eg through subtraction imaging) have been proposed. It is likely therefore that molecular imaging will continue to find increasing application in the management of pituitary tumors just as it already does in other endocrine disorders.

Development of a bespoke phantom to optimize molecular PET imaging of pituitary tumors

BACKGROUND

Image optimization is a key step in clinical nuclear medicine, and phantoms play an essential role in this process. However, most phantoms do not accurately reflect the complexity of human anatomy, and this presents a particular challenge when imaging endocrine glands to detect small (often subcentimeter) tumors. To address this, we developed a novel phantom for optimization of positron emission tomography (PET) imaging of the human pituitary gland. Using radioactive 3D printing, phantoms were created which mimicked the distribution of ¹¹C-methionine in normal pituitary tissue and in a small tumor embedded in the gland (i.e., with no inactive boundary, thereby reproducing the in vivo situation). In addition, an anatomical phantom, replicating key surrounding structures [based on computed tomography (CT) images from an actual patient], was created using material extrusion 3D printing with specialized filaments that approximated the attenuation properties of bone and soft tissue.

RESULTS

The phantom enabled us to replicate pituitary glands harboring tumors of varying sizes (2, 4 and 6 mm diameters) and differing radioactive concentrations (2 ×, 5 × and 8 × the normal gland). The anatomical phantom successfully approximated the attenuation properties of surrounding bone and soft tissue. Two iterative reconstruction algorithms [ordered subset expectation maximization (OSEM); Bayesian penalized likelihood (BPL)] with a range of reconstruction parameters (e.g., 3, 5, 7 and 9 OSEM iterations with 24 subsets; BPL regularization parameter (β) from 50 to 1000) were tested. Images were analyzed quantitatively and qualitatively by eight expert readers. Quantitatively, signal was the highest using BPL with β = 50; noise was the lowest using BPL with β = 1000; contrast was the highest using BPL with β = 100. The qualitative review found that accuracy and confidence were the highest when using BPL with β = 400.

CONCLUSIONS

The development of a bespoke phantom has allowed the identification of optimal parameters for molecular pituitary imaging: BPL reconstruction with TOF, PSF correction and a β value of 400; in addition, for small (< 4 mm) tumors with low contrast (2:1 or 5:1), sensitivity may be improved using a β value of 100. Together, these findings should increase tumor detection and confidence in reporting scans.

Pituitary adenoma & nuclear medicine: Recent outcomes and ongoing developments

In order to explore pituitary adenoma (PA), magnetic resonance imaging (MRI) remains the cornerstone. However, there are some limitations and MRI can be non-conclusive. The development of additional imaging modalities like nuclear medicine explorations may help to confirm PA diagnosis, guide management and follow up. Nuclear medicine uses radiopharmaceuticals for imaging with single photon emission computed tomography (SPECT), or positron emission tomography (PET), coupled to CT scan. Radiopharmaceuticals products target specific cellular elements which allow to explore several biological pathways. Nuclear medicine may also be used for therapeutic purposes and recent developments of approach based on Peptide Receptor Radionuclide Therapy (PRRT) for treatment of aggressive PA and pituitary carcinoma will be reviewed. Several radiotracers have been studied in the context of PA, and the aim of this paper is to discuss their respective performances and clinical interest.