Major Salivary Gland Neoplasms

Pitfalls in the Staging of Cancer of the Major Salivary Gland Neoplasms Elliott R. Friedman, MDa,*, Amit M. Saindane, MDb KEYWORDS Salivary glands Adenoid cystic carcinoma Benign mixed tumor Carcinoma ex pleomorphic adenoma KEY POINTS The major salivary glands consist of the parotid, submandibular, and sublingual glands. Most neoplasms in other subsites in the head and neck are squamous cell carcinoma, but tumors of the salivary glands may be benign or malignant. Surgical treatment differs if the lesion is benign, and therefore preoperative fine-needle aspiration is important in salivary neoplasms. The role of imaging is to attempt to determine histology, predict likelihood of a lesion being malignant, and report an imaging stage. This article reviews the various histologies, imaging features, and staging of major salivary gland neoplasms. ANATOMY AND BOUNDARIES gland is a single contiguous structure, for surgical convenience it has been divided into superficial The major salivary glands consist of the parotid, and deep lobes by the course of the facial nerve, submandibular, and sublingual glands. Submu- which courses lateral to the retromandibular vein. cosal clusters of salivary tissue located in the Most neoplasms arise in the superficial portion of palate, oral cavity, paranasal sinuses, and upper the gland.3 Considerable variation in intraparotid aerodigestive tract are minor salivary glands, and facial nerve anatomy exists, but most commonly are variable in distribution. Primary neoplasms the nerve enters the posteromedial aspect of the arising in the minor salivary glands are staged 1 gland and divides into 2 main trunks before dividing according to the anatomic site of origin. into 5 main branches: temporal, zygomatic, buccal, The parotid gland is the largest of the salivary mandibular, and cervical. The auriculotemporal glands, the bulk of which lies superficial to the nerve, which curves around the mandibular neck, masseter muscle and mandibular ramus and angle embedded in the gland capsule, connects the (Fig. 1). The investing fascia arises from the super- mandibular branch of the trigeminal nerve with the ficial layer of the deep cervical fascia. The deep facial nerve and serves as a potential route of peri- portion of the gland extends through the styloman- neural tumor spread. dibular tunnel into the prestyloid compartment of Accessory parotid tissue is found in approxi- the parapharyngeal space. The stylomandibular mately 20% of the population, anterior to the tunnel is formed by the skull base, posterior margin parotid gland, typically overlying the masseter of the mandibular ramus, and styloid process and muscle between the zygomatic arch and parotid stylomandibular ligament.2 Although the parotid a Department of Diagnostic & Interventional Imaging, University of Texas Health Science Center at Houston, 6431 Fannin Street, MSB 2.130B, Houston, TX 77030, USA; b Department of Radiology and Imaging Sciences, Emory University School of Medicine, BG-22, 1364 Clifton Road Northeast, Atlanta, GA 30322, USA * Corresponding author. E-mail address: [email protected] Neuroimag Clin N Am 23 (2013) 107–122 http://dx.doi.org/10.1016/j.nic.2012.08.009 1052-5149/13/$ – see front matter Ó 2013 Elsevier Inc. All rights reserved. neuroimaging.theclinics.com 108 Friedman & Saindane and deep cervical lymph nodes, particularly IIA nodes. Wharton’s duct, the primary excretory duct, courses anteriorly and superiorly between the gen- ioglossus muscle and sublingual gland to open in the floor of mouth on the sublingual papilla on the side of the tongue.4 The anterior facial vein, which runs along the lateral margin of the submandibular gland, can be used to distinguish exophytic submandibular Fig. 1. CT anatomy of the parotid gland. Abbrevia- masses from extrinsic lesions, such as lymphade- tions: m, muscle; n, nerve; v, vein. nopathy or soft tissue tumors, that arise adjacent to the gland. A mass separated by the facial vein duct.4 In distinction to other salivary glands, be- from the gland must be an extrinsic lesion.6 tween 3 and 24 lymph nodes are found within The sublingual gland is the smallest of the major the parotid gland, almost all within the superficial salivary glands and is situated above the mylo- portion of the gland.5 Parotid lymph nodes drain hyoid muscle, covered by mucosa of the floor into the level IIA and IIB upper cervical chains.2 of the mouth.4 No well-defined capsule exists. The main parotid duct, Stensen’s duct, is approx- Numerous small excretory ducts, the Rivinus imately 7 cm in length, exiting the anterior aspect ducts, open directly into the floor of the mouth. A of the gland, passing horizontally lateral to the large duct, Bartholin duct, may empty into the masseter muscle and medial to the zygomaticus submandibular duct near its orifice.7 Primary major muscle, pierces the buccinator muscle, lymphatic drainage is into level I lymph nodes. and opens on a papilla in the buccal mucosa opposite the second maxillary molar tooth.4 IMAGING WORKUP The submandibular gland occupies most of the submandibular triangle, which is formed by the MR imaging is the preferred cross-sectional im- inferior border of the mandibular body and anterior aging modality to assess a noninflammatory and posterior bellies of the digastric muscle salivary gland mass. Rarely, imaging will be defin- (Fig. 2). The stylohyoid muscle also forms part of itive of a diagnosis (Fig. 3); however, histologic the posterior boundary. The submandibular gland differentiation of neoplasm is generally not pos- is enveloped by a thin fibrous capsule. The stylo- sible on CT or T1-weighted images with or with- mandibular ligament, a reflection of the deep out contrast, although noncontrast T1-weighted cervical fascia, separates the parotid and subman- images are helpful to assess tumor size, margins, 8 dibular glands. The submandibular gland is arbi- depth, and extent. Certain imaging features may trarily divided into superficial and deep lobes suggest malignancy. Bone invasion, perineural based on its extension around the posterior margin of the mylohyoid muscle, which divides the submandibular space from the floor of the mouth. No lymph nodes or large nerves are present within the submandibular gland. Lym- phatic drainage is into IB or submandibular nodes Fig. 2. CT anatomy of the submandibular gland. Fig. 3. Imaging diagnosis of parotid mass. Axial Whether a mass is glandular or nodal may sometimes contrast-enhanced CT shows a well-defined fat- be difficult to determine. If the facial vein comes density mass within the superficial left parotid gland between the gland and the mass, the lesion is nodal. diagnostic of a lipoma (arrow). Major Salivary Gland Neoplasms 109 spread, and deep extension into the parapharyng- assess for perineural spread, bone invasion, or eal space or muscles are highly indicative of meningeal involvement if intracranial extension is malignancy. Although infiltrative margins suggest present. Effacement of fat in the neural foramen malignancy, hemorrhage or inflammatory changes on noncontrast T1-weighted images allows for on CT can create false-positive results for malig- assessment of perineural spread while avoiding nancy (Fig. 4). artifactual degradation that can be present at the Benign and low-grade malignant lesions are skull base on fat-saturated sequences (Fig. 6). likely to exhibit more hyperintense T2 signal, Because almost all salivary gland neoplasms whereas high-grade malignancies tend to have enhance, an important role of contrast in assess- intermediate or lower T2 signal. T2 signal is ing the tumor is distinguishing a T2 hyperintense influenced by intracellular water content and cellu- mass from a cyst. larity. Differentiated benign or low-grade malig- MR imaging can delineate the intraparotid course nant lesions are more likely to produce serous or of the facial nerve and parotid ducts. One technique mucous secretions, which have a high water acquires T1-weighted images through a 3-dimen- content. Unfortunately, although elevated T2 sig- sional (3D) Fourier transform gradient-echo nal suggests that a lesion is not high-grade, it sequence (3D GRASS) using a head and neck coil does not reliably distinguish between benign and at 1.5 T. Optimal results were obtained with a 20- low-grade malignant lesions (Fig. 5). Low-grade cm field of view, 512 Â 288 matrix, flip angle of malignancies, particularly mucoepidermoid and 30, repetition time of 30 ms, and effective echo adenoid cystic carcinomas, may exhibit high T2 time of 4.2 ms, and acquiring 60 sections at 1.5- signal.9 Some benign lesions can have low T2 mm thickness without gaps. Facial nerve and intra- signal, including Warthin tumor, which is the parotid ducts have lower T1 signal intensity than second most common benign tumor of the parotid glandular tissue. Signal intensity within vessels gland, granulomas, and fibrosis. However, for varies with saturation. Curved, orthogonal, and volu- practical purposes, low T2 signal should serve metric reconstructed images of the 3D data allow as a warning that a high-grade lesion may be the facial nerve and its branches to be followed in present.10,11 T1-weighted gadolinium-enhanced most patients from the stylomastoid foramen fat-saturated sequences should be included to through the posterior superior aspect of the gland to the level of the retromandibular vein, and assessment of the parotid ducts at the hilum and in the an- teroinferior portion of the gland.12 This technique is not routine

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