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Carotid space lesions: Added value of 68Gallium-DOTATATE PET/MRI
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This case focuses on a 68-year-old man with history of transient ischemic attack and a 3-cm enhancing right carotid space lesion incidentally detected on outside imaging.
The mass — between the internal jugular vein and internal carotid artery, just below the skull base — was presumed to be paraganglioma.
The patient was referred to our institution for further management.
The clinical team requested 68Gallium-DOTATATE PET/MRI initial staging examination to evaluate for metastatic spread and synchronous paragangliomas prior to treatment with external beam radiation therapy.
Imaging findings
The 68Gallium DOTATATE radiopharmaceutical was injected via IV. The patient was instructed to void at 35 minutes, and then was placed on the PET magnetic resonance scanner.
PET imaging from the abdomen to mid-thighs followed by dedicated imaging of the vertex to upper chest were performed per protocol with magnetic resonance attenuation correction.
During the PET data acquisition, precontrast diagnostic MRI sequences of the abdomen, pelvis, chest, head and neck were obtained, including multiplanar T1-weighted, T2-weighted and diffusion-weighted sequences.
The patient subsequently underwent PET imaging from the vertex to the upper abdomen with axial radial volumetric interpolated breath-hold examination (VIBE) and axial half-Fourier acquisition single-shot turbo spin-echo (HASTE) magnetic resonance sequences for anatomical localization. Dedicated neck MRI also was performed.
The patient was subsequently injected with 7.6 mL gadobutrol (Gadavist, Bayer) and underwent diagnostic postcontrast T1-weighted imaging of the head and neck.
The diagnostic neck MRI aspect of the study demonstrated a heterogeneously T2 hyperintense, well-defined enhancing mass in the right carotid sheath at the skull base extending from the jugular foramen (Figure 1).
Images: Munir Ghesani, MD.
The mass measured 1.7 cm by 1.7 cm by 3.4 cm (anteroposterior by transverse by craniocaudal). The right internal jugular vein was laterally displaced by the mass but remains patent.
The internal carotid artery was anteriorly displaced by the mass without narrowing. The skull base major vessels remained widely patent. The high-resolution constructive interference in steady-state (CISS) images demonstrated the origin of the mass from the pars vascularis of the jugular foramen involving the 10th and/or 11th cranial nerves.
The mass was noted to be confined to the extracranial compartment below the skull base without intracranial extension. The pars nervosa was identified with an intact ninth cranial nerve. No flow voids appeared associated with the mass. No cervical lymphadenopathy was noted.
Of note, 68Gallium-DOTATATE PET/MRI fusion images demonstrated no radiotracer uptake in the mass (Figure 2).
Further, the functional 68Gallium-DOTATATE PET imaging demonstrated no abnormal metabolic uptake in the head and neck, chest, abdomen, pelvis or skeletal structures (Figure 3).
Given the lack of radioisotope uptake, lack of flow voids and predominant intrinsic T2 hyperintense cystic changes, imaging findings were suggestive of a schwannoma of cranial nerve 10.
Subsequently, plasma and urine metanephrines were obtained and values were within normal limits. Therefore, the patient underwent 5 weeks of external beam radiation therapy in an effort to control lesion growth of this right carotid space schwannoma.
Six-month follow-up contrast-enhanced neck MRI demonstrated a heterogeneous, T1 isointense, T2 hyperintense mass centered in the right carotid sheath and extending from the pars vascularis of the right jugular foramen. The mass had increased slightly in size from the prior examination, measuring 1.8 cm by 2 cm by 3.7 cm (anteroposterior by transverse by craniocaudal).
There had been interval change in the enhancement pattern, now with a large area of central nonenhancement consistent with expected postradiation therapy effects.
The right internal carotid artery remained anteriorly displaced by the mass without narrowing. The right internal jugular vein was diminished in caliber and displaced laterally by the mass but remained patent. There was no evidence for intracranial extension. Again, no flow voids were noted in the mass.
Follow-up imaging findings were most compatible with a right carotid space schwannoma, which demonstrated mild and likely transient increase in size compared with 68Gallium-DOTATATE PET magnetic resonance, with new central areas of cystic/necrotic change related to posttreatment effects (Figure 4).
Follow-up imaging, which will be performed in 3 months, will be expected to show eventual decrease in size of the lesion.
Discussion
The carotid space — which extends from the aortic arch to the skull base — traverses the suprahyoid and infrahyoid neck to enter the anterior mediastinum.
Normal anatomic structures within the carotid space include the carotid artery, internal jugular vein, sympathetic plexus, vagus nerve, lymph nodes and remnants of the second branchial cleft. Hence, lesions in the carotid space typically arise from one of the aforementioned structures.
Brachial cleft cysts, although common, typically are nonenhancing and predominantly cystic lesions. Lymph nodes usually are found lateral to the vascular structures of the carotid sheath; therefore, an enhancing mass between the carotid artery and internal jugular vein is less likely to be nodal in origin.
The patient’s tumor most likely arises from one of the neural components of the carotid space. Leading differential considerations should include paraganglioma (carotid body tumor and glomus vagale tumor) and vagal nerve schwannoma. Because biopsy of highly vascular paragangliomas is contraindicated, accurate imaging differentiation between paragangliomas and nerve sheath tumors is highly important.
Paragangliomas (glomus tumors) are slow-growing neoplasms that arise from nonchromaffin paraganglion cells that are found from the base of skull to the urinary bladder. Histologically, main components are lobules or nests of chief cells (type I) surrounded by a single layer of sustentacular cells (type II).
Immunohistochemical examination confirms neuroendocrine differentiation of chief cells (type I) that stain positive for chromogranin-A and synaptophysin. There is a 3:1 female predominance; 25% of paragangliomas are multicentric, and these tend to be familial.
Carotid body tumors account for 60% to 70% of all head and neck paragangliomas. They typically are asymptomatic; however, they may present as a slow-growing pulsatile mass anterior to the sternocleidomastoid muscle at the angle of the mandible at the level of the hyoid bone.
On imaging, these lesions are identified as an avidly enhancing, hypervascular mass in the region of the carotid bifurcation, which splays the external carotid artery anteriorly, and the internal carotid artery posteriorly. They may cause bony erosion and result in a moth-eaten appearance of surrounding osseous structures on CT.
Classically, paragangliomas demonstrate rapid wash-in and wash-out on dynamic contrast studies. MRI findings include T1-hyperintense punctate blood products (“salt”) and serpentine hypointense vascular flow voids (“pepper”) on T2-weighted sequences.
Arteriovenous shunting and an intense tumor blush may be seen on angiography, usually supplied by the ascending pharyngeal artery. On scintigraphy, they demonstrate high uptake with 111In-octreotide (Octreoscan), 123I meta-iodobenzylguanidine (MIBG) SPECT and 131I-MIBG SPECT, as well as on 68Gallium-DOTATATE PET imaging.
Glomus vagale tumors are paragangliomas that arise along the path of the vagus nerve. They are the least common of the head and neck paragangliomas. They typically arise slightly more cephalad in the neck and may be identified as a painless mass located posterior to the carotid arteries.
They result in vocal cord paralysis for 50% of patients. Typical imaging appearance is of a highly vascular enhancing mass that displays the internal and external carotid artery anteriorly, and the internal jugular vein posteriorly. Otherwise, signal characteristics follow that of carotid body tumors. Of note, malignant transformation has been reported in 16% to 19% of glomus vagale tumors and 6% of carotid body tumors.
Schwannomas are slow-growing, benign tumors of Schwann cell origin and are the most common benign peripheral nerve tumor. The vast majority of schwannomas are solitary and sporadic; however, there is an association with neurofibromatosis type 2.
On histopathology, they are composed of spindle cells that demonstrate two growth patterns: Antoni type A and Antoni type B.
The Antoni type A are elongated cells densely packed and arranged in fascicles; palisades form Verocay bodies, whereas Antoni type B cells are less compact and prone to cystic degeneration.
Vagal nerve schwannomas usually are incidentally detected, with hoarseness as the most common presenting symptom. Occasionally, a paroxysmal cough may be elicited by palpating the mass. This is uniquely associated with vagal schwannomas.
On CT, schwannomas demonstrate expansion and remodeling of adjacent bones. On MRI, they are usually hypointense on T1-weighted sequences and hyperintense on T2-weighted sequences due to frequently associated areas of cystic degeneration, with variable degree of enhancement on postcontrast sequences.
Vagal schwannomas displace the internal jugular vein laterally and the carotid artery anteromedially, which allows for appropriate differentiation from schwannomas of the cervical sympathetic chain, which displace both vessels anteriorly or anterolaterally, without separation. The typical “salt-and-pepper” pattern of hypervascular paragangliomas is absent. Further, schwannomas are not avid on somatostatin receptor-based imaging modalities.
Of note, somatostatin receptor-based imaging is the functional imaging modality of choice for the initial diagnosis and management of patients with suspected neuroendocrine tumors, including paragangliomas of the head and neck.
The first radiotracer in clinical use was 111In-octreotide, a long-acting somatostatin analog utilized for this purpose since the 1980s. However, octreotide SPECT imaging (Octreoscan) is associated with several inherent limitations, including low spatial resolution; relatively high physiologic uptake, which limits detection of small lesions; and comparatively high radiation dose to the patient. Similarly, 123I-MIBG SPECT and 131I-MIBG SPECT — which have been developed as the study of choice for patients with suspected pheochromocytoma and paraganglioma — are prone to similar limitations.
In June 2016, the FDA approved 68Gallium DOTATATE, a positron-emitting isotope for imaging of somatostatin receptor-expressing neoplasms.
Compared with Octreoscan and MIBG scintigraphy, 68Gallium-DOTATATE PET dramatically improves spatial resolution and lesion detectability, has shorter acquisition times (less than 2 hours vs. 2 days for Octreoscan and MIBG imaging), and is associated with significantly lower radiation exposure. Further, the affinity of DOTATATE in binding to somatostatin receptor type 2 is approximately 10-fold higher than that of octreotide.
Therefore, the reported sensitivity of 68Gallium-DOTATATE in somatostatin-avid malignancy ranges from 80% to 100%, with a specificity reaching 100%. In addition, 68Gallium-DOTATATE PET provides information on tumor cell receptors status, which has significant impact on clinical management — including targeted radionuclide therapy — by identifying patients who are suitable candidates for peptide receptor-targeted therapy.
Studies have compared semiquantitative MRI with diffusion-weighted imaging (DCE-MRI) to 68Gallium-DOTATATE PET/CT. Although several DCE-MRI parameters — especially flow-related parameters — correlated well with standard uptake values from PET/CT, others correlated poorly or not at all. This suggested added value in hybrid PET/MRI techniques by improving diagnostic power.
In conclusion, the addition of accurate anatomic localization provided by hybrid imaging modalities such as PET/CT and PET/magnetic resonance for both staging and response assessment allows us to take advantage of the synergies offered by combined metabolic, physiologic and structural imaging.
The systematic addition of high-resolution structural magnetic resonance information to PET data in hybrid PET/magnetic resonance examinations — particularly in the head and neck region, as well as other organ systems where superior soft tissue resolution is imperative — helps overcome difficulties in anatomic localization on PET images, may exclude or identify the presence of multiple pathologies and improves scan interpretation without added radiation safety concerns.
In our patient, somatostatin receptor functional imaging with 68Gallium-DOTATATE PET/magnetic resonance was the first modality to alert the clinical team to the lack of a neuroendocrine component, essentially excluding paraganglioma from the differential considerations. Further, the contrast-enhanced diagnostic neck MRI (structural aspect) of the hybrid imaging study allowed for accurate and noninvasive diagnosis of the patient’s right carotid space schwannoma.
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For more information:
Ana M. Franceschi, MD, is a neuroradiology fellow at NYU Langone Medical Center.
Munir Ghesani, MD, FACNM, is assistant professor of radiology and director of PET/CT fellowship at NYU Langone Medical Center in New York. He also is a HemOnc Today Editorial Board Member. He can be reached at munir.ghesani@nyumc.org.
Disclosures: Franceschi and Ghesani report no relevant financial disclosures.