Imaging in Malignancy of the Oral Cavity and Role of PET CT in Squamous Cell Carcinoma of Head and Neck Region

Fig. 2.1

(a and b): Diagram demonstrating anatomical landmarks and important structures in the oral cavity (a) Contents of the oral cavity viewed from an anterior aspect, demonstrating the alveolar ridges, hard and soft palate, palatoglossal fold, tongue, and tonsils. (b) Diaphragmatic representation of the lateral view in the midline, showing superior and inferior recesses of the anterior vestibule, mucosa of the floor of the mouth, the tongue with the mylohyoid diaphragm at the base, the hyoid bone, and the vallecula. The plane passing through the circumvallate papillae and the hard–soft palate junction (curved red dotted line) separates the anterior oral cavity with the posterior oro-pharynx
Posteriorly, the sublingual space freely communicates with the submandibular space, as no fascial boundary separates them. The submandibular space contains lymph nodes (level IB) and the submandibular gland, with the facial artery medially and the facial vein laterally.
The oral vestibule separates the lips and cheeks to form the teeth and the alveolar process by a reflection of the buccal mucosa onto the maxilla and mandible. Adjacent to the alveolar process, the gingivobuccal and the glosso-alveolar sulci are common locations for squamous cell carcinoma of the vestibule and floor of the mouth respectively.
Apart from the identification of the standard anatomy of the structures of the oropharyngeal region, appreciation of the tissue planes, compartments, and their content as displayed using imaging methods is necessary [3].
The submandibular space is of importance in evaluating the extent of nodal disease [submandibular (level 1b) and submental space (level 1a) lymph nodes]. Other components of the space are the facial vein and artery, fat, and the inferior loop of the hypoglossal nerve [1].
The sublingual space, which contains the anterior parts of the hyoglossus muscle; the lingual nerve, artery, and vein; the glossopharyngeal and hypoglossal cranial nerves; the sublingual glands and ducts; the deep component of the submandibular gland; and the submandibular (Wharton’s) duct [1, 4], is not encapsulated by fascia and is often involved in lesions of the floor of the mouth.
Posteriorly, the masticator space and the infratemporal fossa are areas of special attention in the evaluation of disease spread. The masticator space contains the ramus of the mandible with the inferior alveolar nerve within the canal, the masseter, the medial pterygoid, temporalis muscles, the mandibular nerve, and the internal maxillary artery. Although it is a closely confined place, it is open anteriorly toward the buccal space. Posteriorly, it is separated from the parotid space by the fascia and medially related to the parapharyngeal space. The masticator space has a close border with the skull base. the foramina ovale and the spinosum with its contents belong to the space.
There is a slight difference in the terminology of the spaces in the radiological and surgical literature. However, the unambiguous demonstration of an important landmark, the sigmoid notch and demonstration of tumor extension in relation to this landmark, should be a common goal in the evaluation. The spread of malignancy above the level of the sigmoid notch is considered unsuitable for surgical management (T4 b). Important structures in the suprasigmoid notch region are the temporalis muscle and the lateral head of the lateral pterygoid.
The term ‘infratemporal fossa’, sometimes interchangeably used with the masticator space, which is a much broader space consisting of part of the masticator space, but excluding the masseter, the retro-antral buccal space, and part of the parapharyngeal space. Thus, it contains the medial and lateral pterygoid muscles, distal branches of the internal maxillary artery, and branches of mandibular nerve divisions and the rich network of the pterygoid venous plexus. Description of lesions with reference to the infratemporal fossa is invariable in radiology reports in view of its intimate relationship between lesion and the anatomical space and clinical significance of spread of disease to the tissue planes, Identification of nerves and foramina through which nerve passes are important in determining the extent of perineural disease spread.
Anatomical reference to the pterygopalatine fossa is of paramount importance for understanding disease spread from anteriorly located oral, palatal, maxillary, and nasal malignancy to posterior tissue spaces through eight bony canals [5, 6]. The pterygopalatine fossa is bounded anteriorly by the posterior maxilla and posteriorly by the base of the pterygoid plates. The pterygopalatine fissure is the potential space communicating freely with the infratemporal fossa laterally and connecting medially with the nasal cavity through the nasopalatine foramen. There is a free communication of the pterygopalatine fossa with the orbit through the inferior orbital fissure. The foramen rotundum, containing the maxillary nerve, is on the posterior wall of the pterygopalatine fossa, allows communication with the Gasserian ganglion of the trigeminal nerve. Important contents of the pterygopalatine fissure include fat, the sphenopalatine (pterygopalatine) ganglion, the maxillary nerve and its divisions, the para-sympathetic plexus and the distal branches of the internal maxillary artery. The pterygopalatine fossa constitutes one of the major links for disease spread from the oral cavity, the RMT to the infratemporal fossa, and also an important pathway for neural tumor spread [6].
A thorough understanding of sectional anatomy is essential for the interpretation of CT and MR images. Anatomical information is obtained from data sets of CT and MRI are quite comparable for most practical purposes. However, the relative strength and weakness of the individual techniques dictates how each can be used to the best advantage in a given situation. The multidetector computed tomography (MDCT) imaging technique is based on the principle of gray-scale display of the volumetric images obtained from the X-ray attenuation profile of tissues. The range of tissue gray-scale values depends upon the tissue density and the atomic number of the tissues. CT imaging is highly suited to evaluation of the bony structures, for possible erosion or for the 3D reconstruction of bony components. In view of the fact that many tissue planes are defined by fat component, CT examination allows optimal evaluation of soft tissues and fascial boundaries of the subsites. Volumetric CT data sets allow reconstruction of images in the conventional three orthogonal planes and provide a choice of reconstruction in areas of interest, as defined by the imaging specialist. Teeth and metal implants in the bony skeleton and dental fillings lead to streak artifacts, degrading the quality of the images in the adjacent vicinity. On the other hand, MRI techniques display tissues based on hydrogen content. MR images have an intrinsic ability for high tissue contrast, and the ability to evaluate bone marrow. Newer MR techniques provide options for a wide variety of information in the form of angiography, spectroscopy, and perfusion characteristics of the tissues. Relatively high contrast between normal and pathological tissues allows the detection of smaller lesions. Hence, MR techniques are ideal for visualization of small soft-tissue lesions. Gadolinium-enhanced MR images are an essential part of the tumor assessment. Ferromagnetic materials in the form of dental implants or a metal prosthesis cause degradation in the image quality owing to blooming artifacts, which are seen as black (dark) areas without structural details.
Structural details of the oral cavity, tongue, oral cavity, and various compartments are optimally studied in the axial and coronal planes (Fig. 2.2). Sagittal planes add to the information on midline structures such as the anterior part of the oral vestibule, the floor of the mouth, the palate, the dorsum of the tongue, the epiglottis, and the prevertebral spaces. The following CT and MR images illustrate the anatomical structures of subsites, important landmarks such as neuro-vascular bundles (Fig. 2.3) and relationships with muscles and adjacent spaces (Figs. 2.3, 2.4, and 2.5).

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Fig. 2.2

Diaphragmatic coronal image, illustrating the location of the retromolar trigone (RMT) (pink) and its relation to the mandible, pterygomandibular raphe, medial pterygoid, and masseter muscles
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Fig. 2.3

Axial (a and b) and coronal (c and d) contrast-enhanced CT images showing the lingual artery and neurovascular bundle within the tongue. In the coronal image (c) midline septum of the tongue is visible as a hypodense strip (arrow). Myelohyoid muscle (pointer) and anterior belly of diagastic muscle (open arrows) are also seen. Coronal CT image at posterior part of oral cavity (d), show lingual artery (white arrow), Hyoid bone (open arrow), submandibular gland (star) and level I B node (triangle)
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Fig. 2.4

Important relationships of the posterior tongue. (a) The styloglossus muscle (open arrow) and the medial pterygoid (star) are shown in anatomical illustration and on coronal T2-weighted MRI. (b) In the coronal MR image (b) fat planes (solid arrow) are clearly visible between the tongue and medial pterygoid. More anteriorly, the submandibular gland is visible (triangle)
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Fig. 2.5

(a, b) axial T2-weighted and STIR images showing the extrinsic (arrows) and intrinsic (star) musculature of the tongue. (c, d) Coronal and sagittal images showing the intrinsic musculature (star) of the tongue, supported inferiorly by the genioglossus and geniohyoid (long arrows and triangle). Cortical outline of the hard palate is clearly visible. More posteriorly, there is a clear fat plane between the dorsum of the tongue and the epiglottis

2.1.3.2 Pathways of Tumor Spread

Focused systematic gathering of image information is improved by understanding the behavior of the pathology and expected pattern of spread. Some typical patterns at subsites are illustrated herewith. Lesions of the tongue located along the lateral border, spread to sublingual space, extrinsic muscles of the floor of the mouth inferiorly and to the neurovascular bundle medially. Hyoid bone may be involved through the extension via the mylohyoid diaphragm. Occasionally, lesions in the anterior floor of the mouth involve the sublingual/submandibular duct leading to dilated saliva ducts. Posterior tongue lesions can spread along the lateral wall to the tonsillar pillars; in the midline they can involve the vallecula and epiglottis. Lesions of the buccal mucosa can spread laterally to the buccal fat and the subcutaneous tissues, posteriorly to the masseter and infratemporal fossa, and extend medially to involve the gingival mucosa. The gingival lesions can invade into the adjacent bony structures. Subtle involvement of the marrow is well illustrated using contrast-enhanced MR techniques that show high T2 values and contrast enhancement when there is tumor invasion. Perineural extension of the tumor is one of the underestimated aspects of tumor spread. The regional nerves of each space may be involved, showing the caudal and cranial extension of the tumor through perineural spread. The tumor size, extent and the precise estimation of the extent of intended surgical excision can be predicted by image analysis. As a general principle, if more than 30 % of the tissue needs to be removed, a reconstructive procedure is needed [7].
Mandibular invasion requires special reference, as the regional involvement directly translates to mandibular resection of varying degrees. The surgical technique varies from marginal mandibulotomy to mandibular resection [810]. Imaging plays a crucial role in the assessment of bone involvement, as clinical examination has it’s limitations. When the lesion abuts the mandible or involves the superficial cortex, marginal mandibulectomy is performed. Deep bony erosions and involvement of the cancellous bone require a segmental mandibulectomy. Hemimandibulectomy is required and there is involvement of the inferior alveolar canal [8, 10]. In an edentulous mandible, if the tumor components are in the vicinity, a segmental mandibulectomy is a preferred. Mandibular thickness of at least a centimeter is required to maintain stability in the region; this can be confidently estimated using image reconstruction and analysis. Imaging also provides information regarding the remaining uninvolved mandible.

2.1.3.3 Imaging Assessment

After clinical evaluation has been carried out and malignancy is suspected, imaging is essential to determine the morphology of the lesion, the stage of the tumor, and the presence of metastasis. There are number of prognostic factors that determine the survival of patients and affect treatment decisions. The basic prognostic factors are tumor size (T-stage), regional nodal involvement (N-stage), and the presence or absence of distant metastasis (M-stage). The modified TNM classification used today is recommended by the American Joint Society of Cancer Control (AJCC; Table 2.1). Thorough understanding of the biological behavior of the lesion is essential for the radiologist for strategizing imaging requirements. Although uncommon on initial presentation, the presence of distant metastasis and second primary cancers of the upper aerodigestive tract should be evaluated. Imaging plays a definitive role in detecting second primary and evaluating distant metastasis. Imaging methods must be utilized in combination to maximize the strength of each modality [10].

Table 2.1

TNM classification for lesions of the oral cavity and lips
American Joint Committee on Cancer (7th edition) TNM staging for SCC of oral cavity and lips
Tumor
TX – primary tumor cannot be assessed
T0 – no e/o primary tumor
Tis – carcinoma in situ
T1 – ≤2 cm
T2 – >2 cm but ≤4 cm
T3 – >4 cm
T4a – moderately advanced local disease
 (Oral cavity) Tumor invades cortical bone, deep (extrinsic) muscles of tongue, maxillary sinus, or skin of face (Lip) Tumor invades cortical bone, inferior alveolar nerve, floor of mouth, or skin of face
T4b – very advanced local disease
 Tumor involves masticator space, pterygoid plates, skull base, or encases internal carotid artery
Node
NX − cannot be assessed
N0 − no regional lymph node metastasis
N1 − single ipsilateral lymph node, ≤3 cm in greatest dimension
N2
  N2a − single ipsilateral lymph node, >3 cm and ≤6 cm in greatest dimension
  N2b − multiple ipsilateral lymph nodes, ≤6 cm in greatest dimension
  N2c − bilateral or contralateral lymph nodes, ≤6 cm in greatest dimension
N3 − lymph node(s) >6 cm in greatest dimension
Metastasis
M0 − none
Ml − yes
Reproduced with kind permission of Arya et al. [7]
Tumor thickness has been associated with local recurrence and survival of cancer of the oral tongue. The exact depth of invasion and correlation with survival is not clear; however, several studies have suggested that tumor thickness greater than 4.0 mm significantly increases the risk for regional metastases and, therefore, has a negative impact on survival [11]; thus, treatment is recommended in clinically N0 neck, even in the absence of other high-risk histopathological features. Recently, Patel et al. [12] demonstrated that patients with increased tumor thickness are at a high risk of nodal metastases, supporting the liberal use of elective neck dissection in these patients, despite being clinically negative. In this direction, there is a strong recommendation for sentinel lymph node biopsy (SLNB) for squamous cell carcinoma of the oral cavity to detect early nodal involvement irrespective of the tumor depth or thickness measured.
High-risk histopathological features that influence the risk of local regional recurrence include angio-invasion, lymphatic emboli, and perineural invasion. One of the most significant prognosticators on imaging studies of oral cancer is the presence of the extra-capsular spread (ECS) of lymph node metastases [10, 13]. ECS has been identified as an indicator of poor prognosis; therefore, patients with ECS are commonly treated with adjuvant therapy, including radiotherapy and chemo-radiotherapy.
Imaging provides essential information regarding the respectability of a neoplastic lesion. On one hand imaging methods provide information regarding the detection and extent of the lesion; on the other hand, it also demonstrates the status of vital surrounding structures (arteries, nerves, airway), an information much needed for planning surgery. In special situations, such as in T4a and T4b category lesions, a critical decision has to be made, classifying lesions as resectable or unresectable. Even within the T4a (advanced resectable group), there are critical determinants that define the expected extent of major surgical morbidity and mortality. Ouyang and Branstetter [14] analyzed in an extensive review the literature on imaging to determine the parameters (diagnostic criteria) and accuracy of different modalities for evaluating these critical T4a and T4b factors. The following important information obtained by imaging is evaluated for the prognostication of outcome: arterial encasement, prevertebral fascia involvement, mediastinal infiltration, tracheal and esophageal extension, laryngeal cartilage penetration, pre-epiglottic fat involvement, dural spread, bone (mandible/maxilla and skull base) infiltration, perineural spread, orbital involvement, and brachial plexus invasion [14].
For the oral cavity lesions, bone involvement, perineural spread and the resectability of large lymph nodes in the vicinity of the carotid sheath are most important. For the most part, the studies find MR imaging with higher sensitivity but lower specificity than CT. An ever-increasing role for PET/CT is suggested, especially for distant spread, in view of the unique mechanism of lesion detection based on glucose uptake, using the PET technique.

2.1.3.4 Imaging Modalities

Radiological interpretation and reporting format must conform to the latest classification system and portray meaningful information for clinical colleagues [7]. The essential role of imaging modalities in the head and neck region is to provide accurate staging of malignancy. Plain radiography no longer plays a role in the evaluation of soft-tissue lesions. It may be useful in demonstrating calcifications and gross bone destruction involving the bony structures. Pan tomography plays a role in the initial assessment of mandibular erosions, although MDCT and dentascan have similar, more precise abilities in the detection of bone erosion. MDCT with administration of intravenous contrast medium offers an initial assessment of soft tissue, bone, and mucosal involvement. Clearly, CT is extremely useful when there are clinical limitations such as trismus or pain, limiting satisfactory clinical examination. The diagnostic quality of CT is adversely affected by artifacts caused by dental metals, but a slight modification of the imaging technique sometimes overcomes artifacts related to dental amalgam. MDCT is complemented by the puffed cheek technique (PCT), when a lesion in the oral cavity is suspected. PCT involves voluntary blowing of the oral cavity with air during the CT examination (Figs. 2.6, 2.7, and 2.8). Compliance with the technique is variable, depending on the extent and location of the disease. Good-quality examination is usually possible in small lesions, where the contribution of the technique is maximal. Lesions of buccal mucosa at typical anatomical sites, a diagrammatic illustration of the location of lesions in parts of the oral cavity, and patterns of likely spread are illustrated in Fig. 2.9.

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Fig. 2.6

Diaphragmatic anatomy of the oral cavity at the mid-cavity level, as illustrated by the puffed cheek technique. Parotid ducts open at upper part of vestibule, opening at the level of the second molar (open arrow). Inferiorly and superiorly, the mucosa reflects and merges with the gingiva. The relationship among the myelo-hyoid diaphragm (black arrow), the salivary glands (pink), and the 1A lymph node is illustrated (Triangle)
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Fig. 2.7

Anatomy of the oral cavity with the puffed cheek CT (PCT) technique. (a) 3 D rendered image showing distended cheek. Coronal images demonstrating anatomy at the mid (b), anterior (c), and posterior cavity (d). Arrow in Fig. b points to facial vein. Open arrows in Fig. d show superior and inferior gingivo-buccal sulci
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Fig. 2.8

Anatomy of the oral cavity with the puffed cheek CT technique. Axial images of the lower vestibule (a) and the upper vestibule (b) . There is an incidental right pneumoparotid (open arrow). Arrow in Fig. a point to orbicularis oris; Tringle is Fig. b show focal densities due to zygomaticus major
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Fig. 2.9

Coronal diaphragmatic illustrations showing the location of the lesion in the buccal mucosa. (a) Lesion at the inferior gingivo-buccal sulcus (GBS) showing the pattern of spread. (b) Lesion at the mid-buccal mucosa at the level of the occlusal plane. (c) Lesion at the superior GBS, showing the pattern of spread
This technique is very useful when clinical evaluation is limited or the lesion is posterior or in hidden areas (Fig. 2.10). Asymptomatic, self-limiting pneumoparotid is seen in 17 % of patients using the puffed cheek technique [15]. Other dynamic maneuvers, including the modified Valsalva, open-mouth, and phonation maneuvers, are not particularly useful in the assessment of the oral cavity. The extent and depth of the lesion can be easily be demonstrated (Figs. 2.11, 2.12, and 2.13) on CT and MRI. We find spiral the CT technique with multiplanar reconstruction a practical option, with diagnostic quality images for evaluating regional and distant spread. One of the underused option in MDCT evaluation is the curved reconstruction option, which can facilitate demonstration of subtle bone erosions, provide global view of extent of bone destruction and show relation of bone erosion in relation mandibular canal canal. It had special value in erosions around mid-line (Fig. 2.14) and RMT region (Fig. 2.15). In the RMT region, it also shows involvement of anterior fibers of temporalis (Fig. 2.15b). Another unexplored option of image processing is 3-D rendered images of the oral vestibule, which can show proliferate lesions, lesions in posterior part of oral vestibule, sometimes clinically invisible. Examples of normal appearances in selected part of oral vestibule is shown in Fig. 2.16. MDCT is a comprehensive solution for assessment of soft tissue and bone.

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Fig. 2.10

Lesions located relatively posteriorly in the oral cavity are difficult to examine clinically. However, a well-performed PCT examination clearly reveals the lesion. Axial (a), coronal (b) and sagittal (c) CT images demonstrate the lesion (open arrow) and provide added information regarding relation to bone
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Fig. 2.11

(a, b, c) CT images of a patient with an extensive malignant lesion of the right cheek, the lip demonstrating transmural spread and dermal involvement (star). There is fistulation anteriorly and thickening and infiltration of the platysma (solid arrow). Coronal projection shows the extent of the lesion, involving the superior and inferior GBS
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Fig. 2.12

Axial (a and c) sagittal (b) and coronal (d) CT images showing a well-circumscribed elevated lesion at the buccal aspect of the right cheek. CT examination allows demonstration of the precise depth of the lesion (c) (arrows)
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Fig. 2.13

A submucosal lesion may appear normal clinically. CT examination with PCT in Axial (a) and coronal plane (b), however, demonstrates diffuse thickening of the left buccal mucosa (arrows) in addition to limited relative distension of left side of the buccal cavity
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Fig. 2.14

Sagittal image of CT (a) demonstrates enhancing lesion at floor of mouth (black arrow) adjacent to mandible. Axial bone window (b) shows inner cortical erosion (open arrow) Extent and depth of lesion is best demonstrated by curved reformation (c)
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Fig. 2.15

Axial CT image (a) shows a left oropharyngeal lesion (star) extending to RMT and anterior infratemporal fossa (black arrow). Curved soft tissue reconstruction (b) revels involvement of anterior fibers of temporalis (triangle) and considerable extension along alveolar margin. Curved bone window image (c) shows extent f bone destruction and relation of bone erosion (open arrow) in relation to mandibular canal (arrow)
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Fig. 2.16

3D surface rendered image (a) of interior of the oral cavity showing normal mucosal interface. Image (b) demonstrates detailed view of posterior part of vestibule. Image (c) shows inferior gingivo-buccal region
Our examinations are performed using 64-slice MDCT scanners for the detection of alveolar, buccal, lip, and RMT carcinoma. We routinely use the PCT technique for suspected lesions in the oral cavity and oropharynx. Our protocol also involves imaging of the thoracic cavity up to the level of the diaphragmatic domes. We reconstruct chest data for lungs and mediastinum in axial, sagittal, and coronal planes.
The MDCT protocol involves the acquisition of 5-mm spiral data with and without intravenous contrast medium with the PCT for lesions of the oral cavity. Subsequent reconstruction is performed at 0.625-mm intervals. We display image data in the axial, sagittal, and coronal planes. Dedicated additional curved planes are utilized when there is a requirement for the resolution of specific issues. Occasionally, evaluation of volume rendered images of the PCT technique offer valuable information for better definition of the lesion.
A slightly different technique, cone-beam CT, uses a cone-shaped beam, rather than the fan-shaped beam of X-rays. The X-ray source and detector make one full rotation around the patient’s head and generate “projection data,” similar to MDCT. These data are then processed to generate 3D volumetric data, from which reconstructed images in all three planes can be obtained. Cone-beam CT also has MPR and curved planar capabilities. Its chief advantages are a lower radiation dose compared with MDCT [14], a rapid scan time (comparable with that of MDCT systems), the availability of display modes unique to maxillofacial imaging, and a smaller size and cost than conventional CT units, making it more suitable for use in clinical dental practices [14].
Currently, cone-beam CT is best suited to evaluating osseous structures in the craniofacial area, but MDCT remains preferable for the evaluation of soft-tissue lesions, including tumors [16].
Imaging methods, along with clinical information, provides the basis for the staging of the lesion and further patient management strategy. Although in most instances lesion definition is simple and straightforward, there are special situations where the ability of imaging methods are put to the test.
In view of the multiple management options/decisions with mandibular involvement, many imaging techniques have been evaluated in this context. Apart from conventional techniques, coned beam CT, PET CT, and SPECT imaging have been considered [1721]. SPECT radionuclide imaging has shown higher sensitivity at the cost of very low specificity [22]. Thin-section CT with bone reconstruction appears to be the best choice given the relatively higher sensitivity and specificity for the exclusion of bone involvement [7, 23, 24]. Some surgeons referred to performing periosteal stripping in the context of negative CT/imaging studies. A combination of imaging methods has been proposed to implement the strategy to increase the sensitivity and high negative predictive value [21, 2527].
When the prime concern is the invasion of the deep soft tissue, muscle or nerve or lesions located in organs such as the tongue and soft palate, MRI is more accurate [28]. MRI is inherently well suited to the early detection of soft tissue lesion and demonstration of perineural invasion. The features of perineural spread is to be specially scrutinized in patients with adenoid cystic carcinoma. Thus, enhancing a lingual, alveolar or trigeminal nerve may suggest perineural involvement. Although MRI is less definitive in demonstrating cortical bone involvement, it is very sensitive in assessing bone marrow involvement. Invasion of the mandibular marrow, the hard palate or the skull base are potential areas where MRI provides maximum value. A technical limitation of MRI in the neck is the involuntary motion, especially related to swallowing, limiting utility in evaluation of the larynx. The advancement of technology has partially compensated for this limitation. The overall accuracy of CT and MRI for T-staging is comparable [29]. CT is more accurate in nodal assessment [9] Our MRI examination are performed on a 1.5-T scanner with an eight channel dedicated head–neck coil. Standard T1 (TSE,TR 600-900ms; TE 8-15ms)- and T2 (TSE TR 8000-9600ms; TE 90-120ms)-weighted images, STIR TR 6000-7000ms; TE 70ms, diffusion-weighted imaging (DWI), and contrast-enhanced fat-saturated images are performed. A section thickness of 3–5 mm is optimal in most situations. We use matrix size of 512×512 and FOV of 18-24cm. For gross nodal assessment a section thickness of 5 mm is adequate. The PCT can also be utilized along with MRI. Additional MR techniques for improving the quality of images of the oral cavity involve using spacers within the vestibule of the mouth to separate it from the teeth and the alveolar ridges. MRI is the preferred technique for lesions of the oral tongue, the floor of the mouth, and lesions involving the hard palate or bone marrow. MRI also shows great strength in the early detection of perineural extension and the detection of intracranial structures. Magnetic resonance angiography, perfusion magnetic resonance imaging, and spectroscopy offer additional information. MRI is particularly useful in evaluating the encasement of the carotid arteries. Perfusion-weighted MR imaging is finding increasing utility in evaluating tumor response to therapy.
The role of ultrasound is well established in the evaluation of thyroid disease and the assessment of lymph nodes. It can be used for assessment of depth of tumor invasion in anterior tongue and problem solving tool in the lesion involving cheek. Ease of use, availability, and the non-invasive technique without ionizing radiation makes it an excellent choice. The main limiting factor is the inter-observer variability and lack of clear imaging format, which can be perceived as a limitation by the referring surgeon. Part of the limitation can be addressed with a clear understanding of the requirements and proper communication. Nowadays, there is renewed interest in the ultrasound technique, with the microbubble contrast agents for the examination of target lymph nodes. High-resolution ultrasound with dynamic contrast-enhanced examination of lymph nodes may emerge as a preferred technique in the assessment of early nodal involvement by malignancy and response to chemotherapy. The importance of high-resolution ultrasound imaging in the demonstration of superficial tumor invasion and salivary gland/duct involvement is highlighted by some studies [1]

2.1.3.5 TNM Classification of Oral Cavity Lesions

The TNM classification provides the goal and basis for image interpretation and documentation. Management of oral cancer needs precise nodal staging to delineate the complete extent of the disease. The combination of clinical evaluation, biopsy confirmation, and imaging plays a critical role. As an objective, reproducible modality, CT imaging is the preferred technique for nodal assessment. Equally sensitive input can be obtained with MRI; however, it has low specificity. Ultrasound fails to provide a global view of the extent of lymph node disease. Often, MDCT is the modality that is used for the assessment of primary lesion as well as for nodal assessment. Tongue cancers and floor of the mouth tumors are preferentially evaluated by MRI for staging [3035]. The rest of the oral cavity can be imaged using MDCT puffed cheek evaluation with contrast medium.

2.1.3.6 Neck Node Evaluation

Regional nodal spread is closely linked to the location of the lesion, the vicinity with the midline and histological behavior of the lesion. Tongue cancers usually spread to ipsilateral nodes with the possibility of skip metastasis and contralateral spread. SCC of the oral cavity and gingiva occurs in regional 1A, IB, and 2 nodes. Lesions of the hard palate are not often associated with nodal spread. CT and MR techniques are equally as efficient for nodal assessment. Signs of nodal involvement are increases in size, obliteration of the fatty hilum, increased vascularity, an ill-defined outline, and intranodal necrosis. Ten millimeters or more in an axial section are considered positive for nodal involvement. However, false-positive or false-negative results of 15–20 % are noted in the literature [36]. Ultrasound with fine needle aspiration under ultrasound guidance, contrast-enhanced ultrasound, contrast analysis curves of dynamic CT and MRI have been used for nodal assessment [3739]. Recent meta analyses [37] utilizing ultrasound-guided FNA, contrast-enhanced MRI and CT has shown slight superiority of ultrasound-guided FNA over other techniques. However, in the study patient with clinically negative nodes, the sensitivity of ultrasound FNA was around 48 % [7]. In the evaluation of nodal neck assessment by MRI, the inclusion of DWI increases the sensitivity and specificity to 76 and 86 % respectively [38], an outcome nearly comparable with CT and PET. DWI images provide additional value to improve the confidence level for predicting nodal involvement. PET imaging appears to play a limited role in the neck −ve malignancy, in view of the high number of false-positive examinations [39].
In the T staging of the lips and oral cavity, interpretation of TX, T0, T1, T2, and T3 are unambiguous. (Table 2.1). T4 staging, however, needs subtle imaging input for further categorization and requires more specific elaboration. T4a consists of lesions of the lip in which the tumor invades through the cortical bone, inferior alveolar nerve, floor of the mouth, or skin of the face (i.e., the chin or nose). For lesions of the oral cavity, the tumor invades through the cortical bone, into the deep extrinsic muscle of the tongue (genioglossus, hyoglossus, palatoglossus, and styloglossus), maxillary sinus, or skin of the face. In T4b lesions, the tumor involves the masticator space, the pterygoid plates, or the skull base and/or encases the internal carotid artery.1 Subcategories a and b were introduced based on involvement of vital structures and thus their suitability for surgical resection. T4a implies locally advanced but resectable tumor, while T4b implies a tumor that is not technically resectable, but that is suitable for nonsurgical options such as chemo- or radiotherapy [40, 41].
Most experts consider CT to be superior to MRI at detecting extracapsular nodal extension, and there are reports that imaging criteria such as involvement of intranodal fat or spiculated margins of metastatic disease are reliable indicators [41, 42]. Most studies that have compared the accuracy of CT and MRI for the assessment of the neck have found no significant difference between these two modalities [43]. In studies reporting on the important issue of the accuracy of CT or MRI for the assessment of the N0 neck, specificities and sensitivities of CT and MRI vary considerably [9], but as a general rule, between 40 and 60 % of all occult metastases are found using either CT or MRI.

2.1.3.7 Important Imaging Considerations in Individual Subsite: Mandibular/Maxillary Invasion

In the evaluation of the lesions of oral cavity, certain areas deserve special attention. When the lesion is in close proximity with bone, imaging becomes more challenging. The following locations are considered individually.

2.1.3.8 Mandibular/Maxillary Invasion

The presence of bone involvement and assessment of the extent is the most important input for planning the extent of surgical resection and the demand for reconstructive procedures.
The mandibular cortex may be involved by adherence or direct extension of the tumor. The medullary cavity may be involved in a more extensive manner by a deeply penetrating tumor, in which case there may be far more bone involvement than is apparent on the surface. Generally, bone involvement is underestimated by CT, whereas with MRI and bone scintigraphy with SPECT, the extent of bone involvement is often overestimated [2, 44]. A negative MRI or bone scan in all likelihood excludes mandibular invasion. For bone involvement of the RMT, the sensitivity of CT is approximately 50 % with a negative predictive value of 60 %. However, the positive-predictive value is approximately 90 %. Thus, although the CT scan is accurate when bone erosion is clearly identified, when it is negative, the predictive value is unacceptably low. Therefore, it is an inaccurate indicator of bone invasion at the RMT [45]. CT provides an excellent view of both the soft tissue and the bone of the mandible (Figs. 2.17 and 2.18); however, it has several technical limitations, the most significant being artifact caused by dental metals, which can obscure demonstration of the invasion of the mandibular cortex. In addition, CT may misleadingly detect defects in the cortex resulting from irregularly shaped alveoli or peri-apical disease

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Fig. 2.17

Axial contrast CT images of the oral cavity (a) showing a full-thickness soft-tissue mass (open arrow) with associated cortical bone destruction (b) and Fig. (c) demonstrated multiple, enlarged IB lymph nodes (thin arrow)
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Fig. 2.18

Subtle bone destruction, especially of the maxilla requires more extensive scrutiny. Parasagittal CT reconstruction (a) shows a soft-tissue mass (star) with focal bone destruction. Compete extent of the lesion is seen at the posterolateral aspect of the maxilla in coronal (b) parasagittal (c) bone window reconstruction (solid arrow). Coronal image in soft tissue window (d) demonstrate thickened buccal mucosa (arrows) due to SCC
The diagnostic accuracy of dentascan for mandibular invasion is high, yielding a sensitivity of 95 %, and a specificity of 79 % with a positive predictive value of 87 % and a negative-predictive value of 92 % [46]. Dentascan is, therefore, an accurate method for the preoperative evaluation of mandibular invasion in patients with squamous cell carcinoma of the oral cavity.
it has been conclusively demonstrated that MRI is superior for evaluating the medullary space of the mandible (Fig. 2.19), but inadequate for assessing cortical mandibular invasion. Unless there is frank invasion of the bony cortex, periosteal stripping followed by frozen section examination at the time of surgery is often the most reliable measure for managing borderline cases with suspected bone invasion. Recent studies have shown that technetium Tc 99 m bone scintigraphy in the form of planar views or as SPECT provide a high degree of diagnostic accuracy for mandibular invasion by oral squamous cell carcinoma of the alveolus, in both edentulous and dentate patients.

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Fig. 2.19

Axial (a) and coronal (b) MR examination showing cortical and marrow involvement of the posterior body of the right side of the mandible (open arrow)
Demonstration of bony erosion involving the maxilla is quite similar in principle. MDCT evaluation with multiplanar bone reconstruction is the optimal method for the demonstration of bone erosion. The dentascan is technically superior in showing subtle bony details. However, MRI examination has limited value.

2.1.3.9 Palate

Preoperative imaging of this area is important to assess the invasion of the maxillary sinus, palatal bone, and nasal vault. MDCT is optimal for evaluating this region because it offers a high-resolution image of the palatal and nasal bones, optimally displayed in orthogonal planes. Lateral tumors may present signs of invasion and perineural spread via the palatine or trigeminal neurovascular bundle. Pain or anesthesia may suggest nerve invasion and MRI with gadolinium is recommended to demonstrate enhancement or edema of the nerve. DWI with optimized technical parameters enhances the early detection of perineural spread. The depth of invasion dictates the extent of the surgical resection. Superficial lesions of the palatal mucosa are best managed with a wide surgical resection, including the underlying palatal periosteum. The MRI is ideal for imaging the floor of the mouth because it is accurate in identifying soft tissue extent of the lesion and perineural invasion.

2.1.3.10 Infratemporal Fossa

Extension of the lesion to the infratemporal fossa (ITF) and the masticator space up-stages the lesion to the T4a category. Diagrammatic representation of the pathways of spread to the ITF is shown in Fig. 2.20. The pterygomandibular raphe connects the hamulus of the medial pterygoid to the mandible, and serves as a pathway for the spread of the lesion to the ITF, including bony extension to the maxillary tuberosity. RMT lesions also extend along this anatomical pathway by extending through the pterygomandibular ligament. CT and MRI are equally suited for demonstrating lesion spread. However, CT may be preferred in situations where bone involvement is suspected. T1-weighted, fat-suppressed, contrast-enhanced MRI is the best available modality for demonstrating perineural spread.

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Fig. 2.20

(a) diaphragmatic Illustration showing the location of the lesion at the posterior upper RMT, depicting the pattern of disease spread to the maxilla, pterygopalatine fossa, and infratemporal fossa. (b) axial diaphragmatic illustration showing the location of the lesion in the mandible. The RMT and the pattern of disease spread, as illustrated by the arrows

2.1.3.11 Tongue

Magnetic resonance imaging is the preferred method for assessing tongue lesions. There is a difference between the behavior of squamous cell carcinoma of the oral tongue and dorsum of tongue. Extent of the regional involvement, dictates treatment options and extent of surgery in case of surgical management. Important factors to scrutinize in anterior tongue lesion are the involvement of the neurovascular bundle, invasion of the submandibular duct, the spread of lesions across the midline septum and mandible bone erosion (Figs. 2.21 and 2.22). The posterior tongue has different avenues of extension. Areas to look for are extension to the dorsum of tongue, vallecula, and epiglottis in the midline. Laterally, extension of the lesion to the palatoglossal fold, tonsils, RMT, and ITF are to be sought (Figs. 2.23, 2.24, and 2.25). MR imaging has a special role in demonstrating the depth of tumour invasion. Capability of the multiplanar evaluation makes it easy to obtain optimal imaging planes for optimal assessment. Contrast-enhanced MR provide a more precise information regarding deeper extension, compared to non-contrast T2 images. Depth beyond 9.7 mm is closely associated with the increased regional metastasis, hence unfavorable prognosis [47, 48]. Extension of lesions due to extrinsic tongue muscle has poor prognosis, often requires multiple modality treatment.

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Fig. 2.21

(a) Coronal diaphragmatic illustration showing the lesion in the mid-tongue and the patterns of spread, across the midline, to the salivary gland and floor of the mouth. (b) Diaphragmatic illustration in the sagittal plane showing the lesion in the anterior floor of the tongue and the likely pattern of malignancy illustrated by the red arrows
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Fig. 2.22

Enhancing sublingual lesion with involvement of the ipsilateral half of the tongue is seen on the right side, as demonstrated by coronal (a), axial (b) and sagital (c) images of contrast-enhanced CT. multidetector computed tomography (MDCT). Arrows in Fig. a and b demonstrate the location of lingual artery. Multiple arrows in Fig. c show the margins of the lesion.
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Jun 24, 2017 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Imaging in Malignancy of the Oral Cavity and Role of PET CT in Squamous Cell Carcinoma of Head and Neck Region
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