Renaissance of (18)F-FDG positron emission tomography in the imaging of pheochromocytoma/paraganglioma

Imaging of Pheochromocytomas and Paragangliomas

Pheochromocytomas/paragangliomas are unique in their highly variable molecular landscape driven by genetic alterations, either germline or somatic. These mutations translate into different clusters with distinct tumor locations, biochemical/metabolomic features, tumor cell characteristics (eg, receptors, transporters), and disease course. Such tumor heterogeneity calls for different imaging strategies in order to provide proper diagnosis and follow-up. This also warrants selection of the most appropriate and locally available imaging modalities tailored to an individual patient based on consideration of many relevant factors including age, (anticipated) tumor location(s), size, and multifocality, underlying genotype, biochemical phenotype, chance of metastases, as well as the patient's personal preference and treatment goals. Anatomical imaging using computed tomography and magnetic resonance imaging and functional imaging using positron emission tomography and single photon emission computed tomography are currently a cornerstone in the evaluation of patients with pheochromocytomas/paragangliomas. In modern nuclear medicine practice, a multitude of radionuclides with relevance to diagnostic work-up and treatment planning (theranostics) is available, including radiolabeled metaiodobenzylguanidine, fluorodeoxyglucose, fluorodihydroxyphenylalanine, and somatostatin analogues. This review amalgamates up-to-date imaging guidelines, expert opinions, and recent discoveries. Based on the rich toolbox for anatomical and functional imaging that is currently available, we aim to define a customized approach in patients with (suspected) pheochromocytomas/paragangliomas from a practical clinical perspective. We provide imaging algorithms for different starting points for initial diagnostic work-up and course of the disease, including adrenal incidentaloma, established biochemical diagnosis, postsurgical follow-up, tumor screening in pathogenic variant carriers, staging and restaging of metastatic disease, theranostics, and response monitoring.

Head and Neck Paragangliomas: An Update on the Molecular Classification, State-of-the-Art Imaging, and Management Recommendations

Paragangliomas are neuroendocrine tumors that derive from paraganglia of the autonomic nervous system, with the majority of parasympathetic paragangliomas arising in the head and neck. More than one-third of all paragangliomas are hereditary, reflecting the strong genetic predisposition of these tumors. The molecular basis of paragangliomas has been investigated extensively in the past couple of decades, leading to the discovery of several molecular clusters and more than 20 well-characterized driver genes (somatic and hereditary), which are more than are known for any other endocrine tumor. Head and neck paragangliomas are largely related to the pseudohypoxia cluster and have been previously excluded from most molecular profiling studies. This review article introduces the molecular classification of paragangliomas, with a focus on head and neck paragangliomas, and discusses its impact on the management of these tumors. Genetic testing is now recommended for all patients with paragangliomas to provide screening and surveillance recommendations for patients and relatives. While CT and MRI provide excellent anatomic characterization of paragangliomas, gallium 68 tetraazacyclododecane tetraacetic acid-octreotate (ie, ⁶⁸Ga-DOTATATE) has superior sensitivity and is recommended as first-line imaging in patients with head and neck paragangliomas with concern for multifocal and metastatic disease, patients with known multifocal and metastatic disease, and in candidates for targeted peptide-receptor therapy. Keywords: Molecular Imaging, MR Perfusion, MR Spectroscopy, Neuro-Oncology, PET/CT, SPECT/CT, Head/Neck, Genetic Defects © RSNA, 2022.

Genetic Determinants of Pheochromocytoma and Paraganglioma Imaging Phenotypes

Parallel to the application of new PET radiopharmaceuticals for pheochromocytoma and paraganglioma (collectively named PPGLs) imaging, several studies have increased our understanding on their biology, genetics, metabolomics, and embryologic origin. In this review, we highlight the current relationship between genotypes and molecular imaging phenotypes. Additionally, we summarize the referral guidelines for imaging of PPGL patients with or without knowledge of their genetic background.

Pheochromocytoma/paraganglioma: recent updates in genetics, biochemistry, immunohistochemistry, metabolomics, imaging and therapeutic options

Pheochromocytomas and paragangliomas (PPGLs), rare chromaffin/neural crest cell tumors, are commonly benign in their clinical presentation. However, there are a number of cases presenting as metastatic and their diagnosis and management becomes a dilemma because of their rarity. PPGLs are constantly evolving entities in the field of endocrinology brought about by endless research and discoveries, especially in genetics. Throughout the years, our knowledge and perception of these tumors and their genetic background has greatly expanded and changed, and each new discovery leads to advancement in the diagnosis, treatment and follow-up of PPGLs. In this review, we discuss the recent updates in the genetics, biochemistry, immunohistochemistry, metabolomics, imaging and treatment options of PPGLs.

Metabolic Subtyping of Pheochromocytoma and Paraganglioma by 18F-FDG Pharmacokinetics Using Dynamic PET/CT Scanning

Static single-time-frame ¹⁸F-FDG PET/CT is useful for the localization and functional characterization of pheochromocytomas and paragangliomas (PPGLs). ¹⁸F-FDG uptake varies between PPGLs with different genotypes, and the highest SUVs are observed in cases of succinate dehydrogenase (SDH) mutations, possibly related to enhanced aerobic glycolysis in tumor cells. The exact determinants of ¹⁸F-FDG accumulation in PPGLs are unknown. We performed dynamic PET/CT scanning to assess whether in vivo ¹⁸F-FDG pharmacokinetics has added value over static PET to distinguish different genotypes. Methods: Dynamic ¹⁸F-FDG PET/CT was performed on 13 sporadic PPGLs and 13 PPGLs from 11 patients with mutations in SDH complex subunits B and D, von Hippel-Lindau (VHL), RET, and neurofibromin 1 (NF1). Pharmacokinetic analysis was performed using a 2-tissue-compartment tracer kinetic model. The derived transfer rate-constants for transmembranous glucose flux (K ₁ [in], k ₂ [out]) and intracellular phosphorylation (k ₃), along with the vascular blood fraction (V_b), were analyzed using nonlinear regression analysis. Glucose metabolic rate (MR_glc) was calculated using Patlak linear regression analysis. The SUV_max of the lesions was determined on additional static PET/CT images. Results: Both MR_glc and SUV_max were significantly higher for hereditary cluster 1 (SDHx, VHL) tumors than for hereditary cluster 2 (RET, NF1) and sporadic tumors (P < 0.01 and P < 0.05, respectively). Median k ₃ was significantly higher for cluster 1 than for sporadic tumors (P < 0.01). Median V_b was significantly higher for cluster 1 than for cluster 2 tumors (P < 0.01). No statistically significant differences in K ₁ and k ₂ were found between the groups. Cutoffs for k ₃ to distinguish between cluster 1 and other tumors were established at 0.015 min^-1 (100% sensitivity, 15.8% specificity) and 0.636 min^-1 (100% specificity, 85.7% sensitivity). MR_glc significantly correlated with SUV_max (P = 0.001) and k ₃ (P = 0.002). Conclusion: In vivo metabolic tumor profiling in patients with PPGL can be achieved by assessing ¹⁸F-FDG pharmacokinetics using dynamic PET/CT scanning. Cluster 1 PPGLs can be reliably identified by a high ¹⁸F-FDG phosphorylation rate.

Functional Imaging of Paragangliomas with an Emphasis on Von Hippel-Lindau-Associated Disease: A Mini Review

Few reports have presented data and results on functional (i.e., nuclear medicine) imaging of paragangliomas and pheochromocytomas (PGLs/PHEOs) for von Hippel-Lindau (VHL) patients. Nuclear medicine localization modalities for chromaffin tumors can be specific or nonspecific. Specific methods make use of the expression of the human norepinephrine transporter (hNET) and vesicular monoamine transporters (VMATs) by these tumors. These permit the use of radiolabeled ligands that enter the synthesis and storage pathway of catecholamines. Nonspecific methods are not related to the synthesis, uptake, or storage of catecholamines but make use of the tumors' high glucose metabolism or expression of somatostatin receptors. Consensuses and guidelines suggest that metastatic and sporadic PHEOs/PGLs in VHL patients (as in patients with chromaffin tumors of yet unknown genotype) should be evaluated first with ¹⁸F-dihydroxyphenylalanine (¹⁸F-DOPA) positron emission tomography/computed tomography (PET/CT). The functional imaging of second choice is ¹²³I-metaiodobenzylguanidine (¹²³I-MIBG) for PHEOs in VHL patients. ¹²³I-MIBG, ⁶⁸Ga-DOTATATE/DOTATOC/DOTANOC PET/CT, or ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET/CT can be a second choice of functional imaging for PGLs in VHL patients.

The Evolving Role of Succinate in Tumor Metabolism: An 18F-FDG-Based Study

In recent years, inherited and acquired mutations in the tricarboxylic acid (TCA) cycle enzymes have been reported in diverse cancers. Pheochromocytomas and paragangliomas often exhibit dysregulation of glucose metabolism, which is also driven by mutations in genes encoding the TCA cycle enzymes or by activation of hypoxia signaling. Pheochromocytomas and paragangliomas associated with succinate dehydrogenase (SDH) deficiency are characterized by high ¹⁸F-FDG avidity. This association is currently only partially explained. Therefore, we hypothesized that accumulation of succinate due to the TCA cycle defect could be the major connecting hub between SDH-mutated tumors and the ¹⁸F-FDG uptake profile. Methods: To test whether succinate modifies the ¹⁸F-FDG metabolic profile of tumors, we performed in vitro and in vivo (small-animal PET/CT imaging and autoradiography) experiments in the presence of succinate, fumarate, and phosphate-buffered saline (PBS) in different cell models. As a control, we also evaluated the impact of succinate on ¹⁸F-fluorocholine uptake and retention. Glucose transporter 1 (GLUT1) immunohistochemistry was performed to assess whether ¹⁸F-FDG uptake correlates with GLUT1 staining. Results: Intratumoral injection of succinate significantly increased ¹⁸F-FDG uptake at 24 h on small-animal PET/CT imaging and autoradiography. No effect of succinate was observed on cancer cells in vitro, but interestingly, we found that succinate caused increased ¹⁸F-FDG uptake by human umbilical vein endothelial cells in a concentration-dependent manner. No significant effect was observed after intratumoral injection of fumarate or PBS. Succinate, fumarate, and PBS have no effect on cell viability, regardless of cell lineage. Intramuscular injection of succinate also significantly increases ¹⁸F-FDG uptake by muscle when compared with either PBS or fumarate, highlighting the effect of succinate on connective tissues. No difference was observed between PBS and succinate on ¹⁸F-fluorocholine uptake in the tumor and muscle and on hind limb blood flow. GLUT1 expression quantification did not significantly differ between the study groups. Conclusion: The present study shows that succinate stimulates ¹⁸F-FDG uptake by endothelial cells, a finding that partially explains the ¹⁸F-FDG metabotype observed in tumors with SDH deficiency. Although this study is an ¹⁸F-FDG-based approach, it provides an impetus to better characterize the determinants of ¹⁸F-FDG uptake in various tumors and their surrounding microenvironment, with a special emphasis on the role of tumor-specific oncometabolites.

Implications of SDHB genetic testing in patients with sporadic pheochromocytoma

PURPOSE

Succinate dehydrogenase B (SDHB) associated pheochromocytomas (PHEOs) are associated with a higher risk of tumor aggressiveness and malignancy. The aim of the present study was to evaluate (1) the frequency of germline SDHB mutations in apparently sporadic patients with PHEO who undergo preoperative genetic testing and (2) the ability to predict pathogenic mutations.

METHODS

From 2012 to 2016, 82 patients underwent a PHEO surgical resection. Sixteen were operated in the context of hereditary PHEO and were excluded from analysis. Among the 66 remaining cases, 48 were preoperatively screened for an SDHB mutation. In addition to imaging studies with specific radiopharmaceuticals (¹²³I-MIBG or ¹⁸F-FDOPA) for exclusion of multifocality/metastases, 36 patients underwent ¹⁸F-FDG PET/CT.

RESULTS

From the 48 genetically screened patients, genetic testing found a germline SDHB variant in two (4.2%) cases: a variant of unknown significance, exon 1, c.14T>G (p.Val5Gly), and a most likely pathogenic mutation, exon 5, c.440A>G (p.Tyr147Cys), according to in silico analysis. Structural and functional analyses of the protein predicted that p.Tyr147Cys mutant was pathogenic. Both tumors exhibited moderate ¹⁸F-FDG PET uptake with similar uptake patterns to non-SDHB mutated PHEOs. The two patients underwent total laparoscopic adrenalectomies. Of the remaining patients, 44 underwent a laparoscopic adrenalectomy, and two had an open approach. Pathological analysis of the tumors from patients bearing two germline SDHB variants revealed a typical PHEO (PASS 0 and 2). Ex-vivo analyses (metabolomics, SDHB immunohistochemistry, loss of heterozygosity analysis) allowed a reclassification of the two SDHB variants as probably non-pathogenic variants.

CONCLUSIONS

This study illustrates that SDHx mutational analysis can be misleading, even if structural and functional analyses are done. Surgeons should be aware of the difficulty of classifying new SDHB variants prior to implementing SDHB mutation status into a tailored surgical management strategy of a patient.

18F-FLT PET/CT in the Evaluation of Pheochromocytomas and Paragangliomas: A Pilot Study

(18)F-FDG PET/CT has been proven to be a highly sensitive method for pheochromocytomas/paragangliomas (PHEOs/PGLs) associated with succinate dehydrogenase (SDH) mutations. This finding has been attributed to altered tumor cell metabolism resulting from these mutations and does not provide additional prognostic information to genotype. Therefore, identification of new biomarkers for aggressiveness is needed. A high Ki-67 index was proposed to be an additional prognostic factor. This pilot study aimed to evaluate 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) PET/CT, a PET proliferation tracer, as a potential imaging agent in a series of 12 PHEO/PGL patients with different genetic backgrounds, to compare (18)F-FLT uptake with (18)F-FDG PET/CT, and to evaluate classic factors of aggressiveness.

METHODS

Twelve patients (7 metastatic and 5 nonmetastatic) were prospectively evaluated with (18)F-FDG and (18)F-FLT and followed for at least 2 y after the initial imaging work-up. Uptake was assessed at a lesion level, visually and quantitatively by maximum standardized uptake values (SUVmax) for both tracers. (18)F-FLT uptake was compared with risk factors known to be linked with a poor prognosis in PGLs (SDHB-mutated status, lesion size, dopaminergic phenotype) and with (18)F-FDG uptake.

RESULTS

In 12 patients, 77 lesions were assessed. All lesions had low (18)F-FLT uptake (median SUVmax, 2.25; range, 0.7-4.5). There was no apparent superiority of (18)F-FLT uptake in progressive lesions, and most of the lesions showed a mismatch, with high (18)F-FDG uptake (median SUVmax, 10.8; range, 1.1-79.0) contrasting with low (18)F-FLT uptake.

CONCLUSION

This study suggests that PHEOs/PGLs-even those that progress-do not exhibit intense (18)F-FLT uptake. It provides the first in vivo demonstration that proliferation may not be a major determinant of (18)F-FDG uptake in these tumors. These findings provide new insight into the biologic behavior of PGL and suggest that antiproliferative agents may be suboptimal for treatment of these tumors.

Magnetic resonance spectroscopy of paragangliomas: new insights into in vivo metabolomics

Paragangliomas (PGLs) can be associated with mutations in genes of the tricarboxylic acid (TCA) cycle. Succinate dehydrogenase (SDHx) mutations are the prime examples of genetically determined TCA cycle defects with accumulation of succinate. Succinate, which acts as an oncometabolite, can be detected by ex vivo metabolomics approaches. The aim of this study was to evaluate the potential role of proton magnetic resonance (MR) spectroscopy ((1)H-MRS) for identifying SDHx-related PGLs in vivo and noninvasively. Eight patients were prospectively evaluated with single voxel (1)H-MRS. MR spectra from eight tumors (four SDHx-related PGLs, two sporadic PGLs, one cervical schwannoma, and one cervical neurofibroma) were acquired and interpreted qualitatively. Compared to other tumors, a succinate resonance peak was detected only in SDHx-related tumor patients. Spectra quality was considered good in three cases, medium in two cases, poor in two cases, and uninterpretable in the latter case. Smaller lesions had lower spectra quality compared to larger lesions. Jugular PGLs also exhibited a poorer spectra quality compared to other locations. (1)H-MRS has always been challenging in terms of its technical requisites. This is even more true for the evaluation of head and neck tumors. However, (1)H-MRS might be added to the classical MR sequences for metabolomic characterization of PGLs. In vivo detection of succinate might guide genetic testing, characterize SDHx variants of unknown significance (in the absence of available tumor sample), and even optimize a selection of appropriate therapies.