Учебники / Head_and_Neck_Cancer_Imaging

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CONTENTS

7.1Introduction 129

7.2Normal Anatomy 129

7.3.1Tonsillar Cancer 132

7.3.5Lymphatic Spread 137

7.4Treatment 137

7.5Post-treatment Imaging 138

7.6. Other Neoplastic Disease 139

7.6.1Non-Hodgkin Lymphoma 139

7.6.2 Salivary Gland Tumors 140

7.6.3Other 140 References 142

7.1 Introduction

Head and neck cancer commonly originates from the oropharynx. As in most head and neck sites, squamous cell cancer is the most frequently encountered malignant disease. Cigarette smoking and excessive alcohol consumption are well-known risk factors. The accuracy of pretherapeutic staging is an important factor in the treatment planning of oropharyngeal neoplasms; clinical examination and imaging studies are complementary in precisely evaluating tumor extent. As an adjunct to clinical surveillance, imaging can be used to monitor tumor response and to detect recurrent or persistent disease as early as possible.

R. Hermans, MD, PhD

Professor, Department of Radiology, University Hospitals

Leuven, Herestraat 49, 3000 Leuven, Belgium

7.2

Normal Anatomy

The pharynx is divided in three sections: the nasopharynx lies behind the nasal cavity, the oropharynx behind the oral cavity, and the hypopharynx merging with the proximal oesophagus at the lower level of the cricoid bone, behind the larynx.

The pharyngeal constrictor muscles compose the posterior and lateral wall of the pharynx. They all insert on a midline localised fibrous raphe. This posterolateral wall of the pharynx is a continuous structure, without markings allowing separation in a naso-, oroor hypopharyngeal level.

The soft palate separates the nasopharynx from the oropharynx. On imaging studies, often one uses a line extending the level of the hard palate as demarcation line on the lateral and posterior wall; an alternative is to use a horizontal line through the C1–C2 articulation (Chong et al. 1999). Direct sagittal MR images, or sagittally reformatted CT images are well suited for defining the border between the nasoand oropharynx.

The oropharynx is separated from the hypopharynx by the pharyngoepiglottic folds. The border between the oropharynx and oral cavity is more complex, being ring-like and composed by several structures. The upper part of this ring is formed by the junction between the hard and soft palate; therefore, the hard palate is a structure belonging to the oral cavity, while the soft palate is an oropharyngeal structure (actually only its lower surface – the upper surface belongs to the nasopharynx). A mucosal fold, known as the anterior tonsillar pillar, forms the lateral part of the ring. This mucosal fold marks the anterior border of the tonsillar fossa. The lower part of the ring is formed by a row of small structures on the back of the tongue, the circumvallate papillae. These papillae are on a V-shaped line, with the tip of the V pointing posteriorly. By definition, the posterior third of the tongue (called tongue base) is part of the oropharynx, and not of the oral cavity (Hermans and Lenz 1996).

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The soft palate is a complex structure, composed of muscles, some fat and lymphoid tissue. Two muscles, arising from the skull base, take part in the formation of the soft palate. The more medial one is known as the levator veli palatini, the more lateral one as the tensor veli palatini. These muscles have an important role during deglutition and phonation. They also form a functional unit with the Eustachian tube; their action opens the Eustachian tube fissure during swallowing and yawning. Dysfunction of the Eustachian tube causes serous otitis, a common finding in nasopharyngeal cancer.

The anterior and posterior tonsillar pillar define the triangular tonsillar fossa. Both pillars are mucosal folds produced by underlying muscular structures. The anterior muscle is the palatoglossal muscle, connecting the soft palate with the tongue base. The posterior muscle is the palatopharyngeal muscle, which takes part in the formation of the muscular pharyngeal wall. Oropharyngeal cancers commonly arise on the anterior tonsillar pillar, using this muscle and the overlying mucosa as a pathway to spread into the soft palate and tongue base. The tonsillar fossa harbours the palatine tonsil, consisting of encapsulated lymphoid tissue. The palatine tonsil is one of the major tonsils in the lymphoid ring of Waldeyer; the other major tonsils are the lingual tonsil in the tongue base, and the pharyngeal tonsil in the roof of the nasopharynx.

The pit between the tongue base and the free edge of the epiglottis is divided by a median mucosal fold running from the base of the tongue to the epiglottis; this fold is known as the glossoepiglottic fold, and separates both valleculae. The valleculae are part of the oropharynx. Anatomically, the lingual side of the free epiglottic margin is the posterior border of the valleculae. For staging purposes, it is important to know that the entire epiglottis is considered as part of the larynx.

Underneath the vallecular floor a laryngeal fat plane is present, known as the preepiglottic fat plane; this fatty tissue is sometimes used by oropharyngeal tumours to dive into the larynx, an extension that is occult to the examining clinician.

The different subsites of the oropharynx,important for staging purposes, are summarized in Table 7.1.

The largest part of the tongue base is composed of muscles, both intrinsic and extrinsic muscles. On axial imaging, these intrinsic muscular fibres may produce a relative high density in the tongue base on CT images, which should not be confused with an infiltrating mass lesion; the regular and symmetric appearance of these normal muscles allows avoidance of this pitfall.

R. Hermans

Table 7.1. Subsites within the oropharynx (UICC 2002)

Anterior wall

–Base of tongue (posterior to circumvallate papillae or posterior third)

–Vallecula

Lateral wall

–Tonsil

–Tonsillar fossa and tonsillar (faucial) pillars

–Glossotonsillar sulci

Posterior wall

Superior wall

–Inferior surface of soft palate

–Uvula

The lingual artery is the main arterial supply to the tongue. It is a branch of the external carotid artery, arising just distal from the superior thyroid artery and proximal to the facial artery. Three segments can be distinguished (Portugalli et al. 2003):

•A proximal segment, running anterosuperior from the external carotid artery, entering the tongue base just above the hyoid bone, medial to the hyoglossal muscle. The hypoglossal nerve crosses the origin of the lingual artery, but runs lateral to the hyoglossal muscle.

•A middle segment, running along the medial side of the hyoglossal muscle.

•A branching distal segment, dividing into the deep lingual artery and sublingual artery.

The lingual nerve, a branch of the third portion of the trigeminal nerve, enters the tongue at a higher level than the lingual artery; this nerve enters the tongue from the parapharyngeal space, lateral to the stylohyoid and hyoglossal muscle. The lingual nerve contains sensitive and sensory (via the chorda tympani, branch of the facial nerve) fibres for the anterior two-thirds of the tongue. The sensory and sensitive innervation of the tongue base is via the glossopharyngeal nerve.

The lymphoid tissue within the ring of Waldeyer can appear prominent in younger subjects, prolapsing into the pharyngeal lumen. The amount of the lymphoid tissue decreases with age. Patients older than 40 years are not expected to have a significant amount of residual lymphatic tissue, but small tags of tissue may persist. The volume of the tonsils may increase due to an upper respiratory tract infection, but also due to an extranodal lymphoma localisation. A persistent or asymmetric appearing lingual tonsil can cause problems, as differentiation with a malignant tumour is not always possible. Clues, which may help to differentiate,

are air-filled crypts and the presence of small calcifications within the lymphatic tissue. Lymphatic tissue does not invade the deeper structures (Fig. 7.1; see also Fig. 7.9). One should however always stay prudent, and in the case of doubt, biopsies are warranted.

The parapharyngeal space runs from the skull base to the submandibular salivary glands. It essentially consists of fat and is bordered medially by the pharyngeal walls and laterally by the infratemporal space (containing the masticatory muscles). Contrary to the superficial tissues of the oropharynx, which may appear somewhat asymmetric, this deeper lying parapharyngeal space should always appear symmetric; any

asymmetry should be treated with extreme caution (Farina et al. 1999). The parapharyngeal space has a small anterior extension between the oropharyngeal wall and the medial pterygoid muscle; this narrow space is known as the pterygomandibular space, reaching the mandible; it contains the lingual nerve. This small pterygomandibular space is actually a crossroads of several structures belonging to different spaces. The anterior tonsillar pillar, connecting the soft palate to the tongue base, and using the pterygomandibular space as access to the parapharyngeal space, is very close, as well as the retromolar trigonum of the mandible, a small triangular bony space, localised just

behind the third lower molar. The mucosal surface at the level of the retromolar trigone also covers the fibrous pterygomandibular raphe. On the posterior side of this raphe, part of the pharyngeal constrictor muscle originates, while on the anterior side, the buccinator muscle takes its origin, eventually forming the muscular substrate of the cheek.

The parapharyngeal space and its neoplastic pathology is discussed in detail in Chap. 9.

7.3

Squamous Cell Carcinoma

Squamous cell carcinoma is the most frequent malignant tumour of the oropharynx (90%). Most patients complain of sore throat, otalgia or dysphagia; more advanced, invasive tumours may cause severe pain and trismus.

The T staging is based on tumour size, and involvement of adjacent structures (Table 7.2). The most common site of origin of oropharyngeal cancer is the anterior tonsillar pillar.

7.3.1

Tonsillar Cancer

Nearly all tonsillar cancers originate from the anterior tonsillar pillar. These cancers commonly spread

anteroinferiorly to the tongue base, and superomedially to the soft palate, both along the palatoglossal muscle. Anterolateral spread, along the pharyngeal constrictor muscle to the pterygomandibular raphe and retromolar trigone, is also commonly seen (Fig. 7.2). Advanced lesions may invade the mandible, spread along the pharyngeal wall to the hypoand/or nasopharynx, or invade the parapharyngeal space through the pharyngeal wall (Figs. 7.3 and 7.4). Spread to the infratemporal space, with involvement of the muscles of mastication and neurovascular structures in this space may be seen in advanced cases (Fig. 7.5).

Lesions originating from the posterior tonsillar pillar are rare; these may spread inferiorly along the palatopharyngeal muscle.

Table 7.2. T staging of oropharyngeal carcinoma (UICC 2002)

Tis Carcinoma in situ

T1 Tumour ≤ 2 cm in greatest dimension

T2 Tumour > 2 cm but ≤ 4 cm in greatest dimension T3 Tumour measures > 4 cm in greatest dimension T4a Tumour invades any of the following : larynx,

deep/extrinsic muscle of the tongue (genioglossus, hyoglossus, palatoglossus, and styloglossus), medial pterygoid, hard palate, and mandible

T4b Tumour invades any of the following: lateral pterygoid muscle, pterygoid plates, lateral nasopharynx, skull base, or encases the carotid artery

7.3.2

Tongue Base Cancer

Cancer in the tongue base tends to grow silently and deeply, and is often larger than suspected at clinical examination. Tumours may spread along the palatoglossal muscle, cornering the glossotonsillar sulcus, to involve the anterior tonsillar pillar.

Anterior spread into the floor of the mouth and/ or tongue body may occur, along the myloand/or hyoglossal muscle, and/or along the lingual neurovascular bundle (Fig. 7.6). Tongue base cancer may also grow in a retrograde fashion along the lingual vessels towards the external carotid artery (Dubin et al. 2002).Vascular and perineural tumor spread is associated with reduced local and regional tumor

Fig. 7.5 Contrast-enhanced CT image in a patient with a right sided tonsillar cancer. The mass extends into the infratemporal space, parapharyngeal and carotid space (arrows). Encasement of the external carotid artery (white arrowhead), and partial encasement of the internal carotid artery (black arrowhead). Laterally, the mass is bordered by the posterior belly and the digastric muscle (curved arrow) and parotid gland

control and reduced patient survival. A tumor mass with a overall diameter of more than 2 cm on imaging predicts vascular and perineural tumor spread (Mukherji et al. 1996). Infiltration of the normal fatty tissue planes in the base of the tongue, of the fat in the sublingual space, as well as irregular tumor margins are also associated with an increased risk of vascular and perineural tumor spread. Such findings are related to overall tumor bulk.

Spread to the valleculae and piriform sinuses, and into the preepiglottic space may be seen. Retrograde tumor growth sometimes occurs along the styloid musculature (Fig. 7.7).

Extension of a tongue base cancer across the midline usually precludes surgical cure, as one lingual neurovascular pedicle needs to be conserved for sufficient functional recovery to allow safe swallowing (Fig. 7.8).

Differentiation of tongue base cancer from normal lymphoid tissue on the surface of the tongue base may be difficult on imaging studies; the only reliable criterion to diagnose cancer is infiltration of the deeper soft tissue structures (see above) (Fig. 7.9).

Fig. 7.8. Patient with a left-sided tongue base cancer. Axial plain T1-weighted spin echo image shows soft tissue mass growing over the midline (arrowheads), involving the left sublingual space (asterisk) and genioglossus/geniohyoid muscles (arrow)

Fig. 7.9. Axial contrast-enhanced CT image in a patient with a right-sided tongue base cancer (asterisk). The lesion shows the same density as the lingual tonsil (arrowheads), but can be identified because it infiltrates the deeper tissues of the tongue base (arrows, compare to opposite side)

7.3.3

Soft Palate Cancer

Soft palate cancer may spread laterally and inferiorly along the tonsillar pillars. Superior spread

to the nasopharynx occurs in advanced disease (Fig. 7.10). Carcinoma of the soft palate may occasionally spread perineurally along palatine branches of the maxillary nerve (Ginsberg and

DeMonte 1998).