Molecular imaging and theranostic approaches in pheochromocytoma and paraganglioma

Abstract

Pheochromocytomas and their extra-adrenal counterpart paragangliomas (PGLs; together called PPGLs), belong to the family of neural crest-derived tumors. Given the overexpression of a wide variety of specific targets in PPGLs, it seems that these tumors are optimally suited to be imaged by specific radiopharmaceuticals. Thus, theranostics approaches with somatostatin agonists and antagonists are rapidly evolving in the setting of these tumors and may be considered as the next step in the therapeutic arsenal of metastatic PPGLs.

Epidemiology

Pheochromocytoma (PHEOs) and paragangliomas (PGLs; together called PPGLs) belong to the same family of neuroendocrine neoplasms. PPGLs are rare tumors with an annual incidence of about 1–8 patients per million (Stenstrom and Svardsudd 1986). Based on the World Health Organization classification published in 2004 and 2017, the term PHEO should be reserved solely for adrenal PGLs. PHEOs account for about 4% of adrenal incidentalomas with a higher prevalence in an autopsy series (Mantero et al. 2000). Around 5% of solitary PHEOs are hereditary, but the presence of multiple PHEOs or combination of a PHEO and PGL are related to a germline mutation in more of 70% of cases, caused by at least 12–15 well-characterized genes. It is also estimated that about 30–50% of these tumors are initially unrecognized, resulting in serious consequences to the patient, including death from catecholamine excess (McNeil et al. 2000; Platts et al. 1995; Sutton et al. 1981).

Tumor origin and genetic background

Chromaffin cells and sympathetic neurons derive from a common sympathoadrenal (SA) progenitor cell. SA progenitor cells aggregate at the dorsal aorta, where they acquire a catecholaminergic neural fate. Subsquently, the cells migrate ventrally and invade the fetal adrenal cortex to form the adrenal medulla. PHEOs are neuroendocrine tumors that arise from the chromaffin cells of the adrenal medulla. PHEOs can be sporadic or can occur as components of hereditary syndromes. Some important correlations between the gene(s) involved and tumor location have been found as described below:

1. for unilateral PHEOs: succinate dehydrogenase complex subunit B or D (SDHB or SDHD), VHL, and RET.

2. bilateral PHEOs: SDHB, RET, VHL, NF1, MYC associated factor X (MAX), and TMEM127.

3. PHEO with sympathetic PGL: SDHB, SDHD, SDHC, VHL, RET, MAX, and HIF2A.

4. PHEO with head and neck PGL (HNPGL): SDHD, SDHB, SDHC, TMEM127, and VHL.

Major predictors for hereditary PPGLs are: a family history of PGL (especially those related to SDHD mutations); a previous personal history of PPGL; and multifocality or a characteristic syndromic presentation (Jafri et al. 2013; Neumann et al. 2009). Currently, a well-characterized syndromic presentation includes the existence of other tumor types associated with the presence of a PGL (e.g., renal cell carcinoma, gastrointestinal stromal tumor, pituitary adenoma can also be related to SDHx mutations) (Papathomas et al. 2014; Pasini et al. 2008) (Table 1).

Table 1.

Summary of common clinical and biochemical presentations of PPGLs with detectable mutations

	First manifestation	Context at presentation (in index cases)	PHEO at presentation	Additional extra-adrenal PGL	Biochemical phenotype	Malignancy risk
MEN2	MTC	Adult Possible phenotypic features of MEN2 Frequent family history of MTC/PHEO	Uni- or bilateral	Exceptional	EPI	Very low
NF1	Neurofibromas	Adult Phenotypic feature of NF1 Possible family history of NF1	Often unilateral	Exceptional	EPI	Very low
TMEM127	PHEO	Adult Possible family history ofPHEO	Uni- or bilateral	No	EPI	Low
MAX	PHEO	Young adult Frequent family history ofPHEO	Bilateral	Possible	EPI and NE	Moderate
VHL	PHEO/PGL	Young adult Frequent family history of PHEO/PGL	Uni- or bilateral	Moderate (retroperitoneum)	NE	Low
SDHB	PHEO/PGL	Adult Possible family history of PHEO/PGL	Often unilateral	Frequent (retroperitoneum)	NE and/or DA	High
SDHD	PHEO/PGL	Adult Frequent family history ofHNPGL	Uni- or bilateral	Frequent (head and neck)	NE and/or DA	Moderate
SDHC	PHEO/PGL	Adult Possible family history of PPGL	Rare	Frequent (mediastinum)	NE	Low
HIF2A	Congenital polycythemia	Adolescent-young adult females Absence of family history of PGL	Very rare	Almost constant (retroperitoneum)	NE	Moderate

Clinical presentation and diagnosis

PHEOs can be diagnosed clinically by the presence of symptoms of catecholamine oversecretion or cardiovascular attack (stroke, cardiomyopathy, arrhythmia) that could appear spontaneously or caused by various drugs. They can also be revealed by screening patients who harbor mutations in one of the PHEO/PGL susceptibility genes. Hereditary PPGLs occur most often during young adult to mid-adult life.

A thorough family and personal history is important and should include questions regarding endocrine (medullary thyroid carcinoma, PPGLs, primary hyperparathyroidism) and non-endocrine (renal, pancreatic, pituitary, gastrointestinal stromal tumors) malignancies or unexplained sudden “cardiovascular system-related” death in family members, especially of younger age (potentially secondary to an undiagnosed PHEO or PGL).

Symptoms and signs of catecholamine oversecretion should be considered (e.g., sustained or paroxysmal elevations in blood pressure, headache, episodic profuse sweating, palpitations, pallor, and apprehension, or anxiety).

Physical examination should focus on detecting features particularly suggestive of hereditary PHEOs: thyroid nodule, as well as presence of neck lymphadenopathy or any phenotypic feature of MEN2, NF1, or VHL syndromic presentations. There are a few true single pathognomonic findings that will allow for definitive diagnosis of MEN2 by clinical inspection, including the presence of nostalgia paraesthetica (posterior pigmented pruritic patch and nostalgia) in MEN2A with codon 634 mutation or marfanoid body habitus (excessively long arms and legs) and thickened protruding lips due to multiple mucosal neuromas (also present on the distal portion of the tongue and gingiva) in MEN2B. NF1 is characterized by the presence of multiple neurofibromas, café-aulait spot, and Lisch nodules of the iris. VHL patients may also have ocular lesions, consisting of retinal capillary hemangioblastomas.

The diagnosis of PHEOs relies on the presence of elevated plasma or urinary metanephrines. PHEOs present in three well-defined biochemical phenotypes: noradrenergic, adrenergic, and dopaminergic. Norepinephrine-secreting tumors are characteristic for extra-adrenal PGLs, either sporadic, or those with hereditary background, mainly including mutations in succinate dehydrogenase subunits (SDHx), VHL, and fuma-rate dehydrogenase (FH). Epinephrine production in these tumors reflects the presence of the enzyme phenylethanol-N-methyltransferase, which is uniquely found in the adrenal medulla. They can be sporadic or hereditary, including mutations in ret. oncogene (RET), neurofibromatosis type 1 (NF1), and transmembrane protein 127 (TMEM127) genes. A dopaminergic phenotype is characterized by significant elevation of either dopamine or its metabolite methoxytyramine (MTY), or both. Usually, dopamine or MTY elevation is associated with an increase in norepinephrine or normetanephrine, which is commonly seen in patients with SDHx mutations (Table 1).

CT and MRI

On non-contrast computed tomography (CT), PHEOs can demonstrate a variety of appearances. Two-thirds of PHEOs are solid adrenal tumors, while the remainder are complex or have undergone cystic or necrotic changes (Park et al. 2007). Typically, the CT attenuation of a PHEO is similar to soft tissue attenuation and, thus, greater than 10 HU, with most PHEOs having a HU of 20–30 or higher. PHEOs can present with a high attenuation due to the presence of hemorrhage or calcifications. In contrast, necrotic tissue presents with a low attenuation. Typically, a PHEO demonstrates avid enhancement (often greater than 30 HU) (Blake et al. 2004). In addition, enhancement can be heterogenous, or there may be no enhancement due to cystic, necrotic, or degenerated regions within the lesion (Park et al. 2006). On magnetic resonance imaging (MRI), the classic imaging appearance of a PHEO is “light-bulb” bright on T2-weighted imaging. In reality, 30 % of PHEOs demonstrate moderate or low T2-weighted signal intensity (Blake et al. 2004; Mayo-Smith et al. 2001). PHEOs typically demonstrate avid contrast enhancement following the administration of intravenous gadolinium-based contrast material (Mitchell 1992; Varghese et al. 1997).

Molecular imaging

The role of molecular imaging are multiple:

1. Diagnosis of PHEO when metanephrine levels are bordeline in the presence of indeterminate adrenal mass on conventional imaging;

2. Assessement of locoregional extension;

3. Identification of tumor multiplicity, especially in the setting of hereditary forms;

4. Exclusion of metastases, especially in presence of large PHEOs, and in patients with certain genotypes (SDHB, SDHA, SDHD, MAX). Malignancy is defined only by the presence of metastatic lesions at sites where chromaffin cells are normally absent (i.e. bone, liver, lymph nodes).

5. Assessment of potential future aggressive behavior of a PHEO

6. Detection of therapeutic molecular targets

Beyond its localization value, molecular imaging has emerged at the forefront of personalized medicine. Molecular imaging provides unique opportunities for increased characterization of tumors at the molecular level—mirroring ex vivo histological classification, but on a whole-body, in vivo, scale. A number of excellent radiopharmaceuticals have been introduced that target different functional and molecular pathways involved in the pathogenesis and clinical behavior of these tumors (e.g., tumor metabolism, specific transporter/receptor expression), which provides unique in vivo biomarkers. Molecular imaging enables identification of three metabolic-imaging phenotypes: catecholamine metabolism, somatostatin receptors, and glucose uptake-imaging phenotypes (Table 2).

Table 2.

Comparison of different PET radiopharmaceuticals for the localization and staging of PPGLs

PET Radiopharamecuticals	Cellular uptake	Cellular retention	Specificity (%)	Sensitivity sporadic	Sensitivity SDHx
18F-FDA	Norepinephrine transporter (NET)	Neurosecretory vesicles (via VMATs)	100	78	52
18F-FDOPA	Neutral amino acid transporter system L (LATs)	Decarboxylation (AADC) and Neurosecretory vesicles (via VMATs)	>95%	75	61
68Ga-SSA	Somatostatin receptors	Internalization (agonists)	90%	98	99
18F-FDG	Glucose transporters (GLUTs)	Decarboxylation (hexokinase) into 18F-FDG-6P	80%	49	86

Catecholamine metabolism-imaging phenotype

Radiolabeled metaiodobenzylguanidine

Several excellent studies have demonstrated the superiority of the Metaiodobenzylguanidine (¹²³I/¹³¹I–MIBG) compared to the Octreoscan for the diagnosis of PHEOs. The recently introduced hybrid SPECT/CT cameras have increased diagnostic confidence in image interpretation and enhanced sensitivity. However, practical constraints such as long imaging times remain important limitations. To circumvent these limitations and drawbacks, the use of ¹²⁴I–MIBG could provide higher resolution images with potential additional benefits of prospective dosimetry for radionuclide therapy (Hartung-Knemeyer et al. 2012). Very recently, a first-in-human study with ¹⁸F–MFBG (an analog of MIBG) for PET imaging was conducted and also showed promising results (Pandit-Taskar et al. 2017).

¹⁸F–fluorodihydroxyphenylalanine

¹⁸F–fluorodihydroxyphenylalanine (¹⁸F–fluorodopa, ¹⁸F–FDOPA) is taken up through neutral amino acid transporters (mainly LAT1/LAT2), decarboxylated into ¹⁸F–fluorodopamine by aromatic L-amino acid decarboxylase, and concentrated in intracellular vesicles. ¹⁸F–FDOPA PET/CT was found to be a highly sensitive (>90%) and specific (95–100%) imaging modality for PHEO detection (Fiebrich et al. 2009; Fonte et al. 2012; Hoegerle et al. 2002; Ilias et al. 2008; Kaji et al. 2007; Timmers et al. 2009a, b). A special advantage of ¹⁸F–FDOPA over other specific radiopharmaceuticals is the lack of uptake by the healthy adrenal tissue. This would enable a better detection of multiple PHEOs that may coexist in the same gland and are highly suggestive of MEN2 or MAX-related PHEOs (Figs. 1, 2). Tumor uptake is significantly correlated with levels of metanephrines (D.T., unpublished personal observations). ¹⁸F–FDOPA PET/CT is also very sensitive in the diagnosis of extra-adrenal PHEOs that are often present in the setting of SDHx (Timmers et al. 2009a).

Fig. 1.

Molecular imagine and theranostic approaches in pheochromocvtoma and paraganglioma

Right sporadic PHEO. a ¹⁸F–FDG PET/CT, b ⁶⁸Ga-DOTATATE PET/CT, c ¹⁸F–FDOPA PET/CT. Scale bar 5 cm. High ⁶⁸Ga-DOTATATE and ¹⁸F–FDOPA tumor uptake with heterogeneous features which contrast with the moderately avidity for ¹⁸F–FDG (arrows). See the high ⁶⁸Ga-DOTATATE uptake by the normal contralateral adrenal gland

Fig. 2.

Multifocal MAX-related PHEOs. a, c, e, g Contrast-enhanced CT, b, d, f, h ¹⁸F–FDOPA PET/CT. Bilateral PHEO with multiple lesions within the same glands. Scale bar 5 cm

¹⁸F–fluorodopamine and ¹¹C–HED

¹⁸F–Fluorodopamine (¹⁸F–FDA) and ¹¹C–hydroxyephedrine (¹¹C–HED) are the most specific tracers for chromaffin tumors, but are only available in very few centers worldwide. Their use should be restricted to primary PHEOs or sympathetic PGLs. Their role in the detection of metastatic PHEOs is most likely suboptimal.

Somatostatin receptors- imaging phenotype

More recently, PET/CT imaging with ⁶⁸Ga-labeled somatostatin analogues (⁶⁸Ga-DOTA-SSAs) has rapidly evolved, since it does not require a cyclotron to make the radiotracer. All ⁶⁸Ga-DOTA-SSAs (DOTATOC, DOTATATE and DOTANOC) effectively target somatostatin receptor subtype 2 (SST2) (IC50: 2.5 nM; 0.2 nM; and 1.9 nM, respectively), which is the most over-expressed subtype in PGLs.

Moreover, a special advantage of labeled SSAs is that, unlike ¹⁸F–FDOPA, they can be used in the radioactive treatment of these tumors (as theranostic agents). The use of ⁶⁸Ga-DOTA-SSA in the context of primary PHEOs has been less studied, but has shown excellent results in localizing these tumors when they are metastatic or extra-adrenal (Hofman et al. 2015; Kroiss et al. 2015; Naji and Al-Nahhas 2012; Naji et al. 2011; Sharma et al. 2013, 2015). Head-to-head comparison between ⁶⁸Ga-DOTA-SSA and ¹⁸F–FDOPA PET has been performed in only four studies: one retrospective study from Innsbruck Medical University (⁶⁸Ga-DOTATOC in 20 patients with unknown genetic background) (Kroiss et al. 2013), two prospective studies from the NIH (⁶⁸Ga-DOTATATE in 17 and 20 patients) (Hofman and Hicks 2015; Janssen et al. 2015a, b), and one prospective study from La Timone University Hospital (⁶⁸Ga-DOTATATE in 30 patients) (Archier et al. 2015). In these studies, ⁶⁸Ga-DOTASSA PET/CT detected more than ¹⁸F–FDOPA PET/CT, regardless of the genotype and even ¹⁸F–FDG PET/CT in SDHx, metastatic, and head and neck PPGLs (Figure 3).

Fig. 3.

Molecular imaging and theranostic approaches in pheochromocvtoma and paraganglioma

Metastatic SDHB-related PPGL. Previous history of surgery for PHEOs and extradrenal PGLs. The anterior maximum intensity projection images of ⁶⁸Ga-DOTATATE (a), ¹⁸F–FDG (b), ¹⁸F–FDA (c), and ¹⁸F–FDOPA (d) demonstrates ⁶⁸Ga-DOTATATE and ¹⁸F–FDG identifies similar number of lesions and perform superior than ¹⁸F–FDA and ¹⁸F–FDOPA. The right adrenal recurrent pheochromocytoma (marked by green arrows) is missed on ¹⁸F–FDOPA but identified by other radiotracers. All the scans identify liver lesion in dome of right lobe (red arrows), intensity is low in ¹⁸F–FDA, whereas the liver lesion located in inferior right lobe of liver (black arrows) is missed on ¹⁸F–FDA. The localization of skeletal metastases is similar on ⁶⁸Ga-DOTATATE, ¹⁸F–FDG, and ¹⁸F–FDA whereas it is suboptimal on ¹⁸F–FDOPA

It should be noted that SST-based imaging may be somewhat less specific than ¹⁸F–FDOPA, ¹⁸F–fluorodopamine, and ¹¹C–HED in the evaluation of these tumors and could be falsely positive in metastatic lymph nodes due to various cancers, meningiomas, inflammatory processes, and some rare conditions such as fibrous dysplasia (Archier et al. 2015; Hofman and Hicks 2015). In clinical practice, most experienced readers of SSA-based imaging are well aware of these pitfalls (often significantly less avid than typical sites of disease and other distinguishing features explained in the above reference) and unlikely to make these mistakes.

Glucose metabolism-imaging phenotype

Although this imaging modality is not specific, data has clearly shown that ¹⁸F–FDG PET has a superior sensitivity over CT/MRI or ^123/131I–MIBG in the localization of SDHx-related PPGLs, especially those with metastatic disease (Timmers et al. 2012). Although previously a first-line imaging modality in this group of patients, it has recently been surpassed by ⁶⁸Ga-DOTA-SSA PET/CT. Recent studies suggest that the accumulation of succinate due to the TCA defect plays a major role in glucose uptake by PHEO cells, not only via stabilization of hypoxia-inducibal factors (HIFs) proteins (pseudohypoxia) but also by endothelial cells via hormone-like action (Garrigue et al. 2017). In non-SDHx PHEOs, the uptake is usually low to moderate.

Current proposed algorithm for molecular imaging in the diagnosis and staging of PHEO

Based on the currently available imaging techniques, we propose the following approach to investigate a patient with a PHEO (Table 3):

Table 3.

First-line choice radiopharmaceuticals according to mutational status

	Location	Related other tumor conditions	First-choice radiopharmaceutical
MEN2	Adrenal	MTC, parathyroid adenomas or hyperplasia	18F-FDOPA
NF1	Adrenal	Neurofibromas, peripheral nerve sheath tumors and gliomas	18F-FDOPA
TMEM127	Adrenal	RCCs	18F-FDOPA
MAX	Adrenal	None reported	18F-FDOPA
VHL	PHEO/PGL	RCCs and CNS emangioblastomas Pancreatic and testicular tumors	18F-FDOPA
SDHB	PHEO/PGL	GISTs, and RCCs Pituitary adenomas	68Ga-DOTATATE
SDHD	PHEO/PGL	GISTs, RCCs and pituitary adenomas	68Ga-DOTATATE
SDHC	PHEO/PGL	GISTs	68Ga-DOTATATE
HIF2A	PHEO/PGL	Somatostatinomas	18F-FDOPA

1. For diagnosis, the specificity provided by functional imaging techniques using ¹⁸F–FDOPA PET/CT or ⁶⁸Ga-DOTA-SSA is superior to anatomical imaging.

2. For detecting additional tumor sites (multifocality, metastases), functional imaging techniques are superior to anatomical imaging. Based on the most recent studies, both SDHx and non-SDHx PPGLs, especially metastatic ones, are better visualized by ⁶⁸Ga-DOTA-SSA than ¹⁸F–FDOPA PET/CT or even ¹⁸F–FDG PET/CT in SDHx-tumors.

3. For patients with a high risk of developping tumors limited to the adrenal glands with potential multifocality (i.e., RET, MAX), ¹⁸F–FDOPA PET/CT should be used as the first-line approach due to the low tracer uptake by extranodular adrenal tissue.

Theranostic approaches

Therapeutic nuclear medicine (“radionuclide therapy,” “targeted internal,” or “molecular” radiotherapy) is defined as a radiation therapy that uses local, loco-regional, or systemic administration of radionuclides to achieve a transfer of radiation energy to a pathological target tissue, and, by this means, exert a destructive effect on culprit tissue. Most often, tissue-specific radionuclide delivery is performed via a targeting biomolecule, which exploits the targeting of radiation through transporters, receptors, or antigens on tumor cells. A complex set of factors determine the success (or failure) of this approach, such as affinity of the carrier molecule, radionuclide–carrier complex binding stability, pharmacokinetics, biodistribution, or radiosensitivity of the tumor cells.

Theranostics encapsulates the integration of diagnostics and therapeutics in individualized disease management. In the context of nuclear medicine, it refers to the use of molecular targeting vectors, (e.g., peptides) labeled with either diagnostic radionuclides (e.g., positron or gamma emitters) or with therapeutic radionuclides. The prime example of theranostics in nuclear medicine is the use of ¹²³I–MIBG imaging, while MIBG labeled with iodine-131 (¹³¹I–MIBG) is used for molecular radiotherapy. Intense uptake of ¹²³I–MIBG by different tumor sites indicates a high expression of norepinephrine transporter on tumor cells that can, therefore, be targeted with ¹³¹I–MIBG. ¹²³I–MIBG is still recommended for determining if a patient is eligible for molecular radiotherapy with ¹³¹I–MIBG. In metastatic PPGLs, ¹³¹I–MIBG provides symptomatic relief in approximately 75% of cases, and a partial radio-logical response in about 30%, with relatively limited side effects. Administration of higher activities is associated with increased response rates but also increased toxicity (Gonias et al. 2009). As the transport of MIBG by the norepinephrine transporter is a saturable event, the specific activity of the preparation may have important effects on both the efficacy and safety of the theranostic agent. Using a solid labeling approach (Ultratrace®), noncarrier-added radiolabelled MIBG has been efficiently produced. Ultratrace® (Iobenguane ¹³¹I) is currently being evaluated at a high fixed dosage activity. The next step will be to evaluate if an individual patient dosimetry leads to a better outcome. PRRT using ¹⁷⁷Lu-DOTA-Octreotate is another example of a theranostic approach. Eligibility to PRRT is based on ⁶⁸Ga-DOTA-Octreotate or Octreoscan uptake by tumors. ¹⁷⁷Lu-DOTAOctreotate has shown excellent results in the treatment of progressive metastatic midgut neuroendocrine tumors in the Netter-1 study (Strosberg et al. 2017). ¹⁷⁷Lu-DOTA-Octreotate will be rapidly implemented into the therapeutic arsenal for PPGLs (Zovato et al. 2012), especially for inoperable and metastatic tumors (Kong et al. 2017). Recent reports have shown excellent results with SST antagonists. According to these observations, it is possible that SST antagonists may be considered as the next step for peptide-based internal radiotherapy in metastatic PPGL patients.