suprahyoid head & neck tumors

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Suprahyoid Head & Neck Tumors


Blake A. Johnson

Advances in surgical techniques for headand neck neoplasms mandate that imagersbe familiar with the compartmental anatomyof the head and neck. They must also befamiliar with the various forms of pathologythat may affect this important region. Thecervical fascia divides the soft tissuessubjacent to the cranial vault intocompartments, within which pathologicprocesses (especially neoplasms andinfections) may spread without resistance.Knowledge of the structures inherent to thesecompartments provides the imager with anaccurate basis for generating a differentialdiagnosis for a lesion occupying one ofthese spaces. Tumors may expand to involveadjacent spaces, and some types of lesionstypically span several compartments.

In this section, we will review the anatomicand imaging features of the head and neck.

Common head and neck neoplasms, as wellas some uncommon lesions will also bediscussed.

IMAGING TECHNIQUESComputed tomography (CT) continues to bean important modality for evaluating thehead and neck structures. Fat interposedbetween muscle and other soft tissuesprovides excellent contrast in most patients.CT exploits this natural contrast, and doesan excellent job of defining lymph nodes. CTalso characterized nodes with regard to thepresence or absence of central necrosis.Despite these features, magnetic resonanceimaging (MRI) is the imaging modality ofchoice for most lesions in the head andneck. MRI provides unique advantages,including lack of ionizing radiation, superiorsoft tissue contrast, multiplanar capabilities(without repositioning or reformatting) andcharacterization of flow. Disadvantages ofMR include longer acquisition times. This isfrequently problematic in head and necktumor patients who may have difficulty withcontrol of secretions while lying supine foran extended period of time. Fast techniquespartially alleviate this problem. T2-weightedfast spin echo (FSE) images drasticallyreduce imaging time, which minimizesmotion artifact, while providing highresolution images. The conspicuity of lymphnodes and some primary tumors on fatsuppressed fast spin echo images is oftensuperior to enhanced T1 weightedsequences. Some lesions, such asneurofibromas, are more conspicuous onthe enhanced T1 weighted sequences. Fat

suppressed T2 weighted fast spin echo andpost contrast T1 weighted sequencesimproved the detection and delineation ofhead and neck tumors, especially when usedin combination. Enhanced images areinvaluable for detecting perineural spread ofmalignant disease. This is particularlyimportant in cases of squamous cellcarcinoma, adenoid cystic carcinoma,lymphomas, minor salivary gland tumorsand other sarcomas, which have a highproclivity for perineural infiltration. Favoredroutes of spread include the second andthird divisions of the fifth cranial nerve andthe facial nerve. A coronal enhancedsequence employing fat saturationtechnique provides excellent images forevaluation perineural spread. Thin section(3 mm or less) imaging to interrogate theanatomy of interest provides the requisitesensitivity for detecting pathology. Multipleplanes allow for spatial mapping of a lesionand assessment for involvement of adjacentstructures. T1-weighted sagittal and T2-weighted fat-suppressed FSE axial andcoronal images provide a basic study. Ifgadolinium is administered, additionalsequences may include pre- and post-contrast T1-weighted axial, and post-contrast coronal (with fat-saturation)images.

Currently, high performance gradients areavailable, allowing even faster acquisitiontimes. Widespread use of gradients strengthsup to 27 milliTesla / meter will occur overthe next decade. They are currently availablein the Siemens Vision, General Electric EchoSpeed, Philips ACS-NT and the Picker Edgesystems. Echo planar images may beobtained in seconds, which alleviates imagedegradation due to motion. Even patientswho are unable to suppress swallowing ormoving for extended periods of time whileassuming the supine position can be imagedwith this technique. Ultrafast imagingcapabilities also decrease the need forsedation in pediatric patients.

Proton MR spectroscopy may provideadditional specificity when evaluating forrecurrent tumor and/or differentiatingcarcinoma from adjacent uninvolvedmuscle. This technique provides anoninvasive means for interrogating themetabolic components of the soft tissues ofthe neck. Clinical trials are required toevaluate the specificity afforded by thispromising modality for the evaluation ofbenign and malignant tumors of theextracranial head and neck.

The MR signal characteristics of extracranialhead and neck neoplasms, in addition to thearchitectural features assessed in multipleplanes, provide information which increasesspecificity over CT. Nonetheless, becausethe imaging appearance for many of thelesions encountered in the head and neckmay be nonspecific, clinical informationprovides important input for generating anaccurate differential diagnosis. Considera-tion of patient age, clinical findings, andduration of symptoms is important for theimager as well as the clinician.

COMPARTMENTALANATOMYThe three layers of the deep cervical fasciaform partitions which define anatomicspaces and contain structures of thevascular, lymphatic, and aerodigestivesystems. The superficial, middle, and deeplayers which define the suprahyoidcompartments provide useful anatomiclandmarks for accessing pathologicchanges. These include the prevertebral,retropharyngeal, carotid, parotid,masticator, parapharyngeal, and thepharyngeal mucosal spaces. The contents ofthese compartments of the head and neckare summarized in Table 1.

Pathologic processes which arise in one ofthese compartments will expand within itbefore violating the fascial boundary andextending into the adjacent space or theskull base. An expanding lesion will alsodistort and/or displace the adjacentcompartments in a predictable fashion,which provides the imager with informationregarding the site of origin. An important,and easily recognized, compartment toassess for displacement is theparapharyngeal space. Most lesionsinvolving this space invade it from theadjacent compartment. Fat within thiscompartment provides excellent contrastwith the adjacent soft tissues on computedtomography and magnetic resonanceimaging The manner in which this space isdisplaced renders information regarding thelesion site of origin of the offending lesion.

PATHOLOGYThe type of lesions which arrive in the deepspaces of the head and neck are predictablydetermined by the structures which normallyreside within them. Localization of the site oforigin of a mass is thus a vital component ofthe imaging assessment. The compartment

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of origin as well as the imagingcharacteristics provide important clues to theidentity of a tumor. Other important featuresto analyze include the extent of the lesion,involvement of adjacent vascular, neural,soft tissue and osseous structures,compromise of the airway if present, andadenopathy. These features impacttreatment planning and prognosis.Perineural spread and extension to the skullbase should also be excluded. Specialattention to cranial nerves V, VII and IX - XIIis warranted, as they traverse the skull baseand suprahyoid neck. Enhanced imagesmaximize sensitivity for evaluating cranialnerve involvement.

As described above, the manner in whichthe parapharyngeal space is displacedprovides a key indicator as to the site oforigin of an adjacent mass. It is uncommonfor a tumor to arise within theparapharyngeal space, but occasionally, aminor salivary gland tumor, neuroma or aparaganglioma will arise within this space. Alesion primary to the parapharyngeal spacewill generally be surrounded by fat. Thepresence of parapharyngeal fat between alesion in this location and the deep lobe ofthe parotid gland helps localize it asextraparotid in origin. It is much morecommon for the parapharyngeal space to

be displaced by a lesion originating withinone of the adjacent compartments.

A lesion arising within the pharyngeal mu-cosal space displaces the parapharyngealspace posterolaterally. The most commonpharyngeal mucosal space neoplasm issquamous cell carcinoma. (which frequentlyarises within the Fossa of Rosenmuller, orlateral recess.) MR provides superior softtissue contrast relative to CT, allowing formore accurate assessment of the mucosalspace. Superficial lesions may be difficult todetect with either modality, thuspharyngoscopy is a vital component of thetumor survey. This is especially true forpatients who have been treated, aspharyngoscopy has proven more sensitivefor the detection of local recurrence than CTor MR. The primary advantage of crosssectional imaging is evaluating for deepextension, which cannot be assessed viadirect visualization of the mucosa.

Other lesions which may arise in this spaceinclude lymphoma and minor salivary glandtumors. A benign, midline nasopharyngealcyst which is encountered in up to 4% ofscanned patients is the Tornwaldt's cyst. It isclinically important only if it becomesinfected. Table 2 summarizes commonlesions encountered in the suprahyoidspaces.

A lesion arising within the masticator spacewill tend to displace the parapharyngealspace posteromedially as it expands. Themost common lesions intrinsic to themasticator space are odontogenicinfections, which are encountered morefrequently than neoplasms in thiscompartment. Sarcomas, lymphoma andsquamous cell carcinoma represent the mostcommon tumors arising within this space.

A lesion arising within the parotid spacewill impact the parapharyngeal space if itinvolves the deep lobe. Such lesions willdisplace the parapharyngeal spaceanteromedially as they expand. There shouldbe no fat interposed between the lesion andthe parotid gland when the mass ariseswithin the gland, and it should displace thecarotid artery posteriorly. As the massexpands medially, it will extend into thestylomandibular notch, expanding it as thelesion enlarges. The most common lesionsto arise within the parotid gland includebenign mixed tumor (pleomorphicadenoma), which characteristically has alobulated, well-defined margin.

Malignancies within the parotid gland aremuch less common, and are seen withgreater frequency in the smaller salivaryglands. The most frequently encounteredsalivary gland malignancy ismucoepidermoid carcinoma. When multiplelesions are present, consider Warthin´stumor, acinic cell carcinoma, intraparotidnodal metastases, NHL, and benignlymphoepithelial lesions associated withAIDS. MR imaging characteristics contributeto differentiating these lesions, althoughdistinguishing a benign from a malignantprimary parotid neoplasm is not alwaysreliable. In general, carcinomas tend toinvade the adjacent muscles of mastication,and show enhancement followingadministration of a gadolinium chelate.Pleomorphic adenomas often have alobulated contour, enhance followingcontrast administration, and do not infiltrateadjacent muscles. Warthin´s tumors usuallydo not enhance, and display heterogeneoussignal on long-TR images.

Lesions primary to the carotid space(sometimes referred to as the retrostyloidparapharyngeal space) will tend to displacethe parapharyngeal fat anterolaterally.Expansion of the lesion will also result inanterior and lateral displacement of thestyloid process. The most common lesionsencountered within the carotid space areschwannomas and glomus tumors. Thelatter will often be characterized by punctateflow voids, a finding which is less consistentin smaller lesions (under 2 cm). Thesetumors usually displace the internal carotidartery anteriorly. If there is posterior

Table 1: Contents of the deep spaces of the head and neck

COMPARTMENT CONTENTS

Pharyngeal mucosal space MucosaConstrictor muscles Lymphoid tissueMinor salivary glandsPharyngobasilar fascia

Parapharyngeal space FatVesselsMandibular nerve branches

Masticator Space Muscles of masticationMandible Inferior alveolar nerve

Parotid Space Parotid glandFacial nerveVessels (ECA*, RMV**)

Carotid Space Internal carotid arteryInternal jugular veinCranial nerves IX, X, XILymph nodesSympathetic plexus

Retropharyngeal space FatLymph nodes

Prevertebral Space Vertebral artery and veinBrachial plexusPhrenic nervePrevertebral & Scalene musclesVertebral body

*ECA: External carotid artery, **RMV: Retromandibular vein

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displacement of the carotid artery, considera deep lobe parotid lesion or a lesion in theparapharyngeal space.

A mass arising within the retropharyngealspace displaces the parapharyngeal spaceanterolaterally. The pharyngeal mucosalspace will be displaced ventrally, and theprevertebral muscles are compressedagainst the vertebral column. While theinfrahyoid portion of the retropharyngealspace contains no lymph nodes, thesuprahyoid segment does. Thus, a nodalpattern of disease is generally found in thiscompartment. Infection is the most commoncategory of pathology effecting theretropharyngeal space, often secondary toextracapsular extension from involvednodes. Nodal involvement by squamous cellcarcinoma or lymphoma represent the mostfrequently encountered neoplasm. Proximityto the airway and the difficulty in clinicallyevaluating nodal disease in this space makeit an important imaging consideration. Inaddition, this compartment is a potentialconduit for the spread of disease betweenthe neck and the mediastinum.

Prevertebral space lesions will alsodisplace the pharyngeal mucosal spaceanterolaterally, accompanied by ventraldisplacement of the prevertebralmusculature. The latter distinguishesprevertebral origin lesions from those which

arise within the retropharyngeal space. Themost common prevertebral space lesion isabscess secondary to adjacent osteomyelitis.Medistatic lesions involving the vertebralcolumn may also extent into the prevertebralspace. Primary bone tumors are much lesscommon.

CONCLUSIONEvaluation of head and neck lesion requiresconsideration of the space of origin, which islocalized by determining the center of thelesion. Assessing the effect of the mass onthe parapharyngeal space helps localize thesite of origin. When dealing with posteriormasses, the interaction with theparapharyngeal space as well as theprevertebral musculature can helpdistinguish retropharyngeal fromprevertebral masses. Evaluation of the skullbase is critical, to exclude extension toinvolve the osseous structures, and to ruleout perineural extension. Careful evaluationof the cranial vault to exclude intracranialextension is also important, especially whenskull base involvement or perineural spreadis present. Because MR allows multiplanarimaging with superior contrast resolution, itis the imaging modality of choice forevaluating these tumors. Computedtomography serves as an adjunctivemodality to assess for and characterize bone

involvement, and to evaluate tumoralcalcifications.

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Table 2. Lesions of the suprahyoid head & neck

COMPARTMENT LESIONS

Pharyngeal mucosal space Squamous cell carcinoma (SCCA)Lymphoma (NHL)Minor salivary glandsTornwaldt´s cyst

Parapharyngeal space Rare: minor salivary gland tumorUsually secondarily involved

Masticator Space SarcomaLymphomaSquamous cell carcinomaRhabdomyosarcoma

Parotid Space Pleomorphic adenomaWarthin´s tumorMucoepidermoid carcinomaAdenoid cystic carcinomaMetastases: SCCA, melanoma

Carotid Space Glomus tumorSchwannoma Nodal metastases (SCCA)(Vascular lesions)

Retropharyngeal space Nodal metastases (SCCA, NHL, others)Direct invasion (SCCA)(Infection, vascular lesions, tortuous ICA)

Prevertebral Space Vertebral body tumorNeural tumors (schwannoma, neurofibroma)Inflammatory or degenerative spine dz)(Vascular lesions)

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[23] van den Brekel MWM, Stel HV, Castelijns JA,et al. Cervical Lymph Node Metastasis: Assess-ment of Radiologic Criteria. Radiology 1990;177:379-384.

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Spin-Echo Images, AJR 1995; 164:981-987

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suprahyoid head & neck tumors

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